l2tp: enable automatic module loading for l2tp_ppp

When L2TP is configured as a module, requests for L2TP sockets do not result
in the l2tp_ppp module being loaded.  Fix this by adding the appropriate
MODULE_ALIAS to be recognized by pppox's request_module() call.
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_ppp.c

@@ -1845,3 +1845,4 @@ MODULE_AUTHOR("James Chapman <jchapman@katalix.com>");
 MODULE_DESCRIPTION("PPP over L2TP over UDP");
 MODULE_LICENSE("GPL");
 MODULE_VERSION(PPPOL2TP_DRV_VERSION);
+MODULE_ALIAS("pppox-proto-" __stringify(PX_PROTO_OL2TP));