batman-adv: Use includes instead of fwdecls

While it can be slightly beneficial for the build performance to use
forward declarations instead of includes, the handling of them together
with changes in the included headers makes it unnecessary complicated and
fragile. Just replace them with actual includes since some parts (hwmon,
..) of the kernel even request avoidance of forward declarations and net/
is mostly not using them in *.c file.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/gateway_client.h

@@ -9,12 +9,11 @@
 
 #include "main.h"
 
+#include <linux/netlink.h>
+#include <linux/seq_file.h>
+#include <linux/skbuff.h>
 #include <linux/types.h>
-
-struct batadv_tvlv_gateway_data;
-struct netlink_callback;
-struct seq_file;
-struct sk_buff;
+#include <uapi/linux/batadv_packet.h>
 
 void batadv_gw_check_client_stop(struct batadv_priv *bat_priv);
 void batadv_gw_reselect(struct batadv_priv *bat_priv);