libexport.a: Enable IPv6 support in hostname.c

If --enable-ipv6 is specified when building nfs-utils, libexport's
host_foo() helpers can now return both IPv4 and IPv6 addresses.

This means IPv6 presentation addresses and IPv6 DNS resolution
results are handled properly in the mountd cache and /etc/exports,
but does not yet enable IPv6 mountd listeners.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>