batman-adv: Keep interface_tx as local function

interface_tx is not used outside of soft-interface.c and thus doesn't
need to be declared inside soft-interface.h

Signed-off-by: Sven Eckelmann <sven@narfation.org>
diff --git a/net/batman-adv/soft-interface.c b/net/batman-adv/soft-interface.c
index b8d3f248..0fc997e 100644
--- a/net/batman-adv/soft-interface.c
+++ b/net/batman-adv/soft-interface.c
@@ -553,7 +553,7 @@
 	return 0;
 }
 
-int interface_tx(struct sk_buff *skb, struct net_device *soft_iface)
+static int interface_tx(struct sk_buff *skb, struct net_device *soft_iface)
 {
 	struct ethhdr *ethhdr = (struct ethhdr *)skb->data;
 	struct bat_priv *bat_priv = netdev_priv(soft_iface);