libnetlink: introduce rtnl_listen_filter_t

There is no functional change with this commit. It only prepares the next one.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
diff --git a/ip/ipnetns.c b/ip/ipnetns.c
index 438d59b..019f954 100644
--- a/ip/ipnetns.c
+++ b/ip/ipnetns.c
@@ -43,6 +43,7 @@
 static int have_rtnl_getnsid = -1;
 
 static int ipnetns_accept_msg(const struct sockaddr_nl *who,
+			      struct rtnl_ctrl_data *ctrl,
 			      struct nlmsghdr *n, void *arg)
 {
 	struct nlmsgerr *err = (struct nlmsgerr *)NLMSG_DATA(n);