deps: allow crypto/x509 to use syscall
deps: allow crypto/x509 to use syscall

Allows the Windows part of CL 5700087 to land.

I had build tested CL 5753060 (which allows
crypto/x509 to use cgo and io/ioutil), and
didn't spot any errors on Windows.

Turns out I was wrong.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5753065

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go