mkimage: make it easier to run syntax checkers on grub-mkimagexx.c

mkimage: make it easier to run syntax checkers on grub-mkimagexx.c

This makes it so you can treat grub-mkimagexx.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 -W 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>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>