stubdom: fix build with disabled pv-grub

stubdom: fix build with disabled pv-grub

Today the build will fail if --disable-pv-grub as a parameter of
configure, as the main Makefile will unconditionally try to build a
32-bit pv-grub stubdom.

Fix that by introducing a pv-grub-if-enabled target in
stubdom/Makefile taking care of this situation.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Ian Jackson <iwj@xenproject.org>