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>