makefiles/gnu.inc.mk: fix "extraneous text after 'ifneq' directive"

When trying to compile `examples/hello-world` on my 465 MHz Mendocino
I got

/home/benpicco/dev/RIOT/makefiles/toolchain/gnu.inc.mk:29: extraneous text after 'ifneq' directive

Turns out the version of `make` on Archlinux32 is newer than that in
Ubuntu 22.04, which somehow ignored the extra parenthesis.

Turns out this (t)rusty old machine is still good for writing patches :D

makefiles/toolchain/gnu.inc.mk

@@ -26,7 +26,7 @@ OBJDUMP   ?= $(_OBJDUMP)
 GCC_VERSION := $(shell $(CC) -dumpversion | cut -d . -f 1)
 
 # -fmacro-prefix-map requires GCC 8
-ifneq (8, $(firstword $(shell echo 8 $(GCC_VERSION) | tr ' ' '\n' | sort -n))))
+ifneq (8, $(firstword $(shell echo 8 $(GCC_VERSION) | tr ' ' '\n' | sort -n)))
   OPTIONAL_CFLAGS_BLACKLIST += -fmacro-prefix-map=$(RIOTBASE)/=
 endif