pfkey: fix warning

If CONFIG_NET_KEY_MIGRATE is not defined the arguments of
pfkey_migrate stub do not match causing warning.
Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/key/af_key.c

@@ -2560,7 +2560,7 @@ static int pfkey_migrate(struct sock *sk, struct sk_buff *skb,
 }
 #else
 static int pfkey_migrate(struct sock *sk, struct sk_buff *skb,
-			 struct sadb_msg *hdr, void **ext_hdrs)
+			 const struct sadb_msg *hdr, void * const *ext_hdrs)
 {
 	return -ENOPROTOOPT;
 }