netfilter: nfnetlink_queue: fix missing HW protocol

Locally generated IPv4 and IPv6 traffic gets skb->protocol unset,
thus passing zero.

ip6tables -I OUTPUT -j NFQUEUE
libmnl/examples/netfilter# ./nf-queue 0 &
ping6 ::1
packet received (id=1 hw=0x0000 hook=3)
                         ^^^^^^

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>