efi: Add missing __grub_efi_api attributes

efi: Add missing __grub_efi_api attributes

The commit bb4aa6e06 (efi: Drop all uses of efi_call_XX() wrappers) did
not add some __grub_efi_api attributes to the EFI calls. Lack of them
led to hangs on x86_64-efi target. So, let's add missing __grub_efi_api
attributes.

Fixes: bb4aa6e06 (efi: Drop all uses of efi_call_XX() wrappers)
Reported-by: Christian Hesse <list@eworm.de>
Reported-by: Robin Candau <antiz@archlinux.org>
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Tested-by: Robin Candau <antiz@archlinux.org>
Tested-by: Christian Hesse <list@eworm.de>
Reviewed-by: Peter Jones <pjones@redhat.com>