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 <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/x86/xen/mmu.c

···
 	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)
 {