[PPPOL2TP]: Label unused warning when CONFIG_PROC_FS is not set.

When CONFIG_PROC_FS is not set and CONFIG_PPPOL2TP is set,
we have the following warning in build:
  drivers/net/pppol2tp.c: In function 'pppol2tp_init':
  drivers/net/pppol2tp.c:2472: warning: label
'out_unregister_pppox_proto' defined but not used

This patches fixes this warning by adding appropriate #ifdef.
Signed-off-by: Rami Rosen <ramirose@gmail.com>
Acked-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/pppol2tp.c

@@ -2468,9 +2468,10 @@ static int __init pppol2tp_init(void)
 
 out:
 	return err;
-
+#ifdef CONFIG_PROC_FS
 out_unregister_pppox_proto:
 	unregister_pppox_proto(PX_PROTO_OL2TP);
+#endif
 out_unregister_pppol2tp_proto:
 	proto_unregister(&pppol2tp_sk_proto);
 	goto out;