net: fix parsing literal IPv6 address with zone identifier when using cgo

Parsing literal IPv6 address with zone identifier is already supported
when not using cgo. This change enables it when using cgo too.

Fixes #12241.

Change-Id: I3ed78c9e750e75eff0dae76ba8608df39503cf85
Reviewed-on: https://go-review.googlesource.com/17215


Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>