NTP requires IPv6 again...

["Scott Ellis" <scotte%warped.com@localhost>]

Didn't track down the source of this yet (if we're supposed to change
PLATFORM_HAS_IPV6 based on MKINET6 or not), but I see a build failure with
-current on amd64:

/nbu/source/netbsd/src/external/bsd/ntp/dist/lib/isc/unix/net.c:373:
warning: 'initialize_ipv6pktinfo' defined but not used

This is with mk.conf having MKINET6=no in it.