BUILD: contrib: fix ip6range build on Centos 7

Jarno Huuskonen reported that ip6range doesn't build anymore on
Centos 7 (and possibly other distros) due to "in6_u" not being known.
Using s6_addr32 instead of in6_u.u6_addr32 apparently works fine, and
it's also what the Lua code uses so it should be OK.

This patch may be backported to 1.6.
1 file changed