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

Change-Id: I2ad06647b7b41cf68859820a60eeac2e689ca2e6
Reviewed-on: https://go-review.googlesource.com/109344
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>