Declare the NFLOG pseudo-header ourselves.

Declare the NFLOG pseudo-header ourselves.

It's not specified by a libpcap header that might have a different
layout in different pcap releases, it's specified on the list of
link-layer header types and must remain the same forever (except for
getting additional bits defined), so we don't need to pick it up from
libpcap.

This means we get to use tcpdump's nd_ types; do so.