arch: arm: remove un-necessary inclusion of <string.h>
arch: arm: remove un-necessary inclusion of <string.h>

Remove the inclusion of <string.h>, if CONFIG_INIT_STACKS
is defined, because it is not required anywhere in thread.c.

Signed-off-by: Ioannis Glaropoulos <5921cc8bab7e1d4329f52fd8f6268f9692e3de80@nordicsemi.no>

galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,ldts/zephyr,punitvara/zephyr,Vudentz/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,explora26/zephyr,punitvara/zephyr,explora26/zephyr,galak/zephyr,ldts/zephyr,explora26/zephyr,explora26/zephyr,explora26/zephyr,nashif/zephyr,finikorg/zephyr,punitvara/zephyr,Vudentz/zephyr,finikorg/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,punitvara/zephyr