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/include/linux/if_pppox.h b/include/linux/if_pppox.h
index 29bcd55..397921b 100644
--- a/include/linux/if_pppox.h
+++ b/include/linux/if_pppox.h
@@ -204,7 +204,7 @@
 	struct module	*owner;
 };
 
-extern int register_pppox_proto(int proto_num, struct pppox_proto *pp);
+extern int register_pppox_proto(int proto_num, const struct pppox_proto *pp);
 extern void unregister_pppox_proto(int proto_num);
 extern void pppox_unbind_sock(struct sock *sk);/* delete ppp-channel binding */
 extern int pppox_ioctl(struct socket *sock, unsigned int cmd, unsigned long arg);