This fixes the following vet warnings:

go/types/builtins.go:437: arg call for printf verb %s of wrong type: *go/ast.CallExpr
go/types/builtins.go:598: arg call for printf verb %s of wrong type: *go/ast.CallExpr

Updates #11041

Change-Id: I746d054e8e49b330fbdf961912a98f55dd5f3ff9
Reviewed-on: https://go-review.googlesource.com/26997
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>