resolve: enable protocol family logic for synthesized IPv6

- Enable protocol family logic for IPv6 resolves even when support
for synthesized addresses is enabled.

This is a follow up to the parent commit that added support for
synthesized IPv6 addresses from IPv4 on iOS/OS X. The protocol family
logic needed for IPv6 was inadvertently excluded if support for
synthesized addresses was enabled.

Bug: https://github.com/curl/curl/issues/863
Ref: https://github.com/curl/curl/pull/866
Ref: https://github.com/curl/curl/pull/867