9p: add missing module license for xen transport

The 9P of Xen module is missing required license and module information.
See https://bugzilla.kernel.org/show_bug.cgi?id=198109

Reported-by: Alan Bartlett <ajb@elrepo.org>
Fixes: 868eb122739a ("xen/9pfs: introduce Xen 9pfs transport driver")
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/9p/trans_xen.c

@@ -543,3 +543,7 @@ static void p9_trans_xen_exit(void)
 	return xenbus_unregister_driver(&xen_9pfs_front_driver);
 }
 module_exit(p9_trans_xen_exit);
+
+MODULE_AUTHOR("Stefano Stabellini <stefano@aporeto.com>");
+MODULE_DESCRIPTION("Xen Transport for 9P");
+MODULE_LICENSE("GPL");