Found by pending CL to make cmd/vet auto-detect printf wrappers.

Change-Id: I6b5ba8f9c301dd2d7086c152cf2e54a68b012208
Reviewed-on: https://go-review.googlesource.com/109345
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>