[Fix] Fix compilation warning for non-split keebs after #17423

[KarlK90]

Description

I forgot to add a #ifdef guard for the non-split keyboard case. That resulted in build errors on develop.

Successful builds again

AVR non-split

make adelheid:default
QMK Firmware breakpoint_2022_05_28
Making adelheid with keymap default

avr-gcc (GCC) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Size before:
   text    data     bss     dec     hex filename
      0   16270       0   16270    3f8e adelheid_default.hex

Copying adelheid_default.hex to qmk_firmware folder                                                 [OK]
Checking file size of adelheid_default.hex                                                          [OK]
 * The firmware size is fine - 16270/28672 (56%, 12402 bytes free)

AVR split

make crkbd:snowe
QMK Firmware breakpoint_2022_05_28
Making crkbd/rev1 with keymap snowe

Size before:
avr-gcc (GCC) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

   text    data     bss     dec     hex filename
      0   23784       0   23784    5ce8 crkbd_rev1_snowe.hex

Copying crkbd_rev1_snowe.hex to qmk_firmware folder                                                 [OK]
Checking file size of crkbd_rev1_snowe.hex                                                          [OK]
 * The firmware size is fine - 23784/28672 (82%, 4888 bytes free)

ARM non-split

make acheron/austin:default
QMK Firmware breakpoint_2022_05_28
Making acheron/austin with keymap default

arm-none-eabi-gcc (15:10.3-2021.07-4) 10.3.1 20210621 (release)
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Size before:
   text    data     bss     dec     hex filename
      0   35040       0   35040    88e0 acheron_austin_default.bin

Compiling: platforms/chibios/synchronization_util.c                                                 [OK]
Linking: .build/acheron_austin_default.elf                                                          [OK]
Creating binary load file for flashing: .build/acheron_austin_default.bin                           [OK]
Creating load file for flashing: .build/acheron_austin_default.hex                                  [OK]

Size after:
   text    data     bss     dec     hex filename
      0   35040       0   35040    88e0 acheron_austin_default.bin

Copying acheron_austin_default.bin to qmk_firmware folder                                           [OK]
(Firmware size check does not yet support STM32F072; skipping)

ARM split

make zvecr/zv48/f401:default
QMK Firmware breakpoint_2022_05_28
Making zvecr/zv48/f401 with keymap default

arm-none-eabi-gcc (15:10.3-2021.07-4) 10.3.1 20210621 (release)
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Size before:
   text    data     bss     dec     hex filename
      0   35532       0   35532    8acc zvecr_zv48_f401_default.bin


Size after:
   text    data     bss     dec     hex filename
      0   35532       0   35532    8acc zvecr_zv48_f401_default.bin

Copying zvecr_zv48_f401_default.bin to qmk_firmware folder                                          [OK]
(Firmware size check does not yet support STM32F401; skipping)

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[KarlK90]

CC: @Jpe230