Don't do IPv6 stuff if INET6 isn't defined.

[tcpdump] / tests / msnlb.out

MS NLB heartbeat, host priority: 2, cluster IP: 192.168.100.80, host IP: 192.168.100.82
MS NLB heartbeat, host priority: 1, cluster IP: 192.168.100.80, host IP: 192.168.100.81