libc: add `stddef.h` to the minimal libc
libc: add `stddef.h` to the minimal libc

It is required for using `size_t`.

Signed-off-by: Eduardo Montoya <c6d924925ebf82d772472f7877e771cedf439387@nordicsemi.no>

zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr