include/grub/arm/system.h: Include missing <grub/symbol.h> header

include/grub/arm/system.h: Include missing <grub/symbol.h> header

The header uses the EXPORT_FUNC() macro defined in <grub/types.h> but
doesn't include it, which leads to the following compile error on arm:

../include/grub/cpu/system.h:12:13: error: ‘EXPORT_FUNC’ declared as function returning a function
   12 | extern void EXPORT_FUNC(grub_arm_disable_caches_mmu) (void);
      |             ^~~~~~~~~~~
../include/grub/cpu/system.h:12:1: warning: parameter names (without types) in function declaration
   12 | extern void EXPORT_FUNC(grub_arm_disable_caches_mmu) (void);
      | ^~~~~~
make[3]: *** [Makefile:36581: kern/efi/kernel_exec-sb.o] Error 1

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>