efi: Make EFI PXE protocol methods non-callable

efi: Make EFI PXE protocol methods non-callable

The grub_efi_pxe_t struct definition has placeholders for the various
protocol method pointers, given that they are never called in the code,
and the prototypes have been omitted, and therefore do not comply with
the UEFI spec.

So let's convert them into void* pointers, so they cannot be called
inadvertently.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>