arch: arm64: core: include kernel_arch_func.h to mitigate warning

A warning was promoted to error in twister runs due to implicit
declaration of the function `z_arm64_safe_exception_stack_init()`.

Include `kernel_arch_func.h` in `prep_c.c` to mitigate the warning.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>