Portability fix for commit
9a895462d.
So far as I can find, NI_MAXHOST isn't actually required anywhere by
POSIX. Nonetheless, commit
9a895462d supposed that it could rely on
having that symbol without any ceremony at all. We do have a hack
for providing it if the platform doesn't, in getaddrinfo.h, so fix
the problem by #including that file. Per buildfarm.