xen: export get_phys_to_machine

-tip testing found the following xen-console symbols trouble:

  ERROR: "get_phys_to_machine" [drivers/video/xen-fbfront.ko] undefined!
  ERROR: "get_phys_to_machine" [drivers/net/xen-netfront.ko] undefined!
  ERROR: "get_phys_to_machine" [drivers/input/xen-kbdfront.ko] undefined!

with:

  http://redhat.com/~mingo/misc/config-Mon_Jun__2_12_25_13_CEST_2008.bad

arch/x86/xen/mmu.c

@@ -136,6 +136,7 @@ unsigned long get_phys_to_machine(unsigned long pfn)
 	idx = p2m_index(pfn);
 	return p2m_top[topidx][idx];
 }
+EXPORT_SYMBOL_GPL(get_phys_to_machine);
 
 static void alloc_p2m(unsigned long **pp, unsigned long *mfnp)
 {