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 <[email protected]>

Author: cfriedt

arch/arm64/core/prep_c.c

@@ -14,6 +14,8 @@
  * initialization is performed.
  */
 
+#include "kernel_arch_func.h"
+
 #include <zephyr/linker/linker-defs.h>
 #include <zephyr/platform/hooks.h>
 #include <zephyr/arch/cache.h>