kernel/smp: Warning cleanup
kernel/smp: Warning cleanup

When CONFIG_SMP is enabled but CONFIG_MP_NUM_CPUS is 1 (which is a
legal configuration, though a weird one) this static function ends up
being defined but unused, producing a compiler warning.

Signed-off-by: Andy Ross <c70f9a6bf6ee0cd69c8af4ce5e6b131945769315@intel.com>

ldts/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,ldts/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,ldts/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr