Fix compiling on GNU/Hurd. [skip appveyor]

[tcpdump] / tests / ipv6_invalid_length_2.out

    1  08:59:14.753767 IP6 [payload+header length 105 > length 104] (invalid) (flowlabel 0x67576, hlim 64, next-header UDP (17) payload length: 65) 2605:bc80:3010:104::8cd3:9ce.45678 > 2600:3c00:e000:19::1.53: [udp sum ok] 34053+ [1au] A? www.tcpdump.org. (56)