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>