Should fix nacl.

Follow-up to
https://golang.org/cl/17936 (fix race) and
https://golang.org/cl/17914 (fix build) for
https://golang.org/cl/16953 (broke the build)

Third time's a charm.

Change-Id: I23930d5cff4235209546952ce2231f165ab5bf8a
Reviewed-on: https://go-review.googlesource.com/17939
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>