xen/efi: efibind: fix MISRA C 2012 Directive 4.10 violation

xen/efi: efibind: fix MISRA C 2012 Directive 4.10 violation

Prevent header file from being included more than once by adding ifndef guard.

In order to be close to gnu-efi code
- for x86_64, use the same guard
- for arm64, that there is no guard in gnu-efi, for consistency,
use a similar format and position to the x86_64 guard

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>