xen: export arbitrary_virt_to_machine

Fixes this build error:

 ERROR: "arbitrary_virt_to_machine" [drivers/xen/xen-gntdev.ko] undefined!
Signed-off-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/x86/xen/mmu.c

@@ -201,6 +201,7 @@ xmaddr_t arbitrary_virt_to_machine(void *vaddr)
 	offset = address & ~PAGE_MASK;
 	return XMADDR(((phys_addr_t)pte_mfn(*pte) << PAGE_SHIFT) + offset);
 }
+EXPORT_SYMBOL_GPL(arbitrary_virt_to_machine);
 
 void make_lowmem_page_readonly(void *vaddr)
 {