include/linux/vgaarb.h: add missing part of include guard

vgaarb.h was missing the #define of the #ifndef at the top for the guard
to prevent multiple #include's from causing re-define errors

Signed-off-by: Doug Goldstein <cardoe@gentoo.org>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>