net: constify some ppp/pptp structs

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index ff954b3..39a21d0 100644
--- a/net/l2tp/l2tp_ppp.c
+++ b/net/l2tp/l2tp_ppp.c
@@ -1768,7 +1768,7 @@
 	.ioctl		= pppox_ioctl,
 };
 
-static struct pppox_proto pppol2tp_proto = {
+static const struct pppox_proto pppol2tp_proto = {
 	.create		= pppol2tp_create,
 	.ioctl		= pppol2tp_ioctl
 };