Fix build issue with z_sys_trace_thread_switched_in

[galak]

commit 4aa4883 (upstream/topic-sensors)
Author: Patrik Flykt patrik.flykt@intel.com
Date: Thu Mar 14 19:42:51 2019 -0600

subsystems: Rename reserved function names

Changed _sys_trace_thread_switched_in to z_sys_trace_thread_switched_in, however we already had a z_sys_trace_thread_switched_in, so this causes build warnings (and possible issues).