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>