arm/xen: Export phys_to_mach to fix Xen module link errors

Various xen drivers fail to link when built as modules with the following
error:

ERROR: "phys_to_mach" [drivers/xen/xen-gntalloc.ko] undefined!
ERROR: "phys_to_mach" [drivers/net/xen-netfront.ko] undefined!
ERROR: "phys_to_mach" [drivers/net/xen-netback/xen-netback.ko] undefined!
ERROR: "phys_to_mach" [drivers/block/xen-blkfront.ko] undefined!

The mfn_to_pfn and pfn_to_mfn functions get inlined in these modules and
those functions require phys_to_mach.  Export the symbol to fix the link
errors.
Signed-off-by: Josh Boyer <jwboyer@fedoraproject.org>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

arch/arm/xen/p2m.c

@@ -27,6 +27,7 @@ struct xen_p2m_entry {
 
 rwlock_t p2m_lock;
 struct rb_root phys_to_mach = RB_ROOT;
+EXPORT_SYMBOL_GPL(phys_to_mach);
 static struct rb_root mach_to_phys = RB_ROOT;
 
 static int xen_add_phys_to_mach_entry(struct xen_p2m_entry *new)