xen: export arbitrary_virt_to_machine

Fixes this build error:

 ERROR: "arbitrary_virt_to_machine" [drivers/xen/xen-gntdev.ko] undefined!

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>