core: make ap_net.h self-contained

We need to include <sys/socket.h> to define 'socklen_t', <sys/types.h>
for 'ssize_t' and "list.h" for 'struct list_head'.

Also, let's include "libnetlink.h" so that we don't need a forward
declaration for 'struct rtnl_handle'.
Signed-off-by: NGuillaume Nault <g.nault@alphalink.fr>

accel-pppd/include/ap_net.h

 #ifndef __AP_NET_H
 #define __AP_NET_H
 
-struct rtnl_handle;
+#include <sys/socket.h>
+#include <sys/types.h>
+
+#include "libnetlink.h"
+#include "list.h"
 
 struct ap_net {
 	struct list_head entry;