netfilter: dup: resolve warnings about missing prototypes

Missing include file causes:

net/netfilter/nf_dup_netdev.c:26:6: warning: no previous prototype for ‘nf_fwd_netdev_egress’ [-Wmissing-prototypes]
net/netfilter/nf_dup_netdev.c:40:6: warning: no previous prototype for ‘nf_dup_netdev_egress’ [-Wmissing-prototypes]
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_dup_netdev.c

@@ -13,6 +13,7 @@
 #include <linux/netfilter.h>
 #include <linux/netfilter/nf_tables.h>
 #include <net/netfilter/nf_tables.h>
+#include <net/netfilter/nf_dup_netdev.h>
 
 static void nf_do_netdev_egress(struct sk_buff *skb, struct net_device *dev)
 {