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/drivers/net/pppoe.c b/drivers/net/pppoe.c
index c07de35..d72fb05 100644
--- a/drivers/net/pppoe.c
+++ b/drivers/net/pppoe.c
@@ -1124,7 +1124,7 @@
 	.ioctl		= pppox_ioctl,
 };
 
-static struct pppox_proto pppoe_proto = {
+static const struct pppox_proto pppoe_proto = {
 	.create	= pppoe_create,
 	.ioctl	= pppoe_ioctl,
 	.owner	= THIS_MODULE,