debug: coredump: Fix build issue with armclang

Building tests/drivers/coredump/coredump_api with armclang gets:

include/zephyr/debug/coredump.h:98:2: error: unknown type name 'uint8_t'
        uint8_t *buffer;
        ^

Fix simply be including <stdint.h> to get uint8_t typedef.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>