drivers: thermal: Mark function as static in x86_pkg_temp_thermal.c

Mark function sys_set_trip_temp() as static in x86_pkg_temp_thermal.c
because it is not used outside this file.

This eliminates the following warning in x86_pkg_temp_thermal.c:
drivers/thermal/x86_pkg_temp_thermal.c:218:5: warning: no previous prototype for ‘sys_set_trip_temp’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>