tcpdump: decode pfsync packets on network interfaces

When print-ip-demux.c was introduced on ee67461e, the pfsync_ip_print
function was missed, causing tcpdump to treat pfsync packets on network
interfaces as an unknown protocol.

MFC after: 1 week
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D42504