xenbus: export xen_store_interface for xenfs

xen_store_interface is needed by xenfs, and xenfs may be a module.

[ Impact: build fix for modular xenfs ]
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

drivers/xen/xenbus/xenbus_probe.c

@@ -64,9 +64,11 @@
 
 
 int xen_store_evtchn;
-EXPORT_SYMBOL(xen_store_evtchn);
+EXPORT_SYMBOL_GPL(xen_store_evtchn);
 
 struct xenstore_domain_interface *xen_store_interface;
+EXPORT_SYMBOL_GPL(xen_store_interface);
+
 static unsigned long xen_store_mfn;
 
 static BLOCKING_NOTIFIER_HEAD(xenstore_chain);