structs under the INET6 #ifdef. Similarly (even though it doesn't seem
to affect the build), conditionalize all IPv4 structs under the INET
#ifdef
This also unbreaks the LINT-NOINET6 tinderbox target on amd64; I have not
verified other MACHINE/TARGET pairs (e.g. armv6/arm).
MFC after: 2 weeks
X-MFC with: r310847
Pointyhat to: jpaetzel
Reported by: O. Hartmann <o.hartmann@walstatt.org>