Fixes build on plan9.

Change-Id: Idbb1e6887c24a873de77c92095198847ed953278
Reviewed-on: https://go-review.googlesource.com/36915
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>