rtnl_link: include "netlink.h" before <linux/rtnetlink.h>

<sys/socket.h> has to be included before <linux/netlink.h> because
the latter used to be incomplete in older kernel headers.

As a local wrapper file called netlink.h was introduced earlier
to workaround this and related portability issues, include it before
<linux/rtnetlink.h> that in turn includes <linux/netlink.h>.

This fixes build on systems with older kernel headers.

* rtnl_link.c: Include "netlink.h" before <linux/rtnetlink.h>.

rtnl_link.c

@@ -31,6 +31,7 @@
 #include "netlink_route.h"
 #include "print_fields.h"
 
+#include "netlink.h"
 #include <linux/rtnetlink.h>
 
 DECL_NETLINK_ROUTE_DECODER(decode_ifinfomsg)