util/grub-module-verifierXX: Enable running standalone checkers

util/grub-module-verifierXX: Enable running standalone checkers

Allow treating util/grub-module-verifierXX.c as a file you can build
directly so syntax checkers like vim's "syntastic" plugin, which uses
"gcc -x c -fsyntax-only" to build it, will work.

One still has to do whatever setup is required to make it pick the
right include dirs, which -I options we use, etc., but this makes
it so you can do the checking on the file you're editing, rather
than on a different file.

Signed-off-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Robbie Harwood <rharwood@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>