drivers: flash: soc_flash_xmc4xxx: Add missing kernel.h header
drivers: flash: soc_flash_xmc4xxx: Add missing kernel.h header

Fixes build of flash_shell sample after commit
a6a4400b8621bc618c60561c5bdd90b2e8ce09e0.
west build -p -b xmc45_relax_kit samples/drivers/flash_shell

Also include stdint.h.

Signed-off-by: Andriy Gelman <9b800b50c068448b0612a5bdc33ac199b44926a7@gmail.com>

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