kernel: Add extern "C" to kernel_struct.h
kernel: Add extern "C" to kernel_struct.h

Add extern "C" to kernel_struct.h to allow linkage in cpp.

Signed-off-by: Marcin Zwierz <13ed11b0216fdaecd12a18710499981d89be9830@hidglobal.com>

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