cmd/fix: print warning to stderr, show provenance

This cgo warning shows up when running "go test" in cmd/fix:

	warning: no cgo types: exit status 1

(It may show up at other times too.)

Warnings belong on standard error, and while we're here,
make it say where it came from.

Change-Id: I3eaba592a219cc6abf2d584762374f9d6ec5135e
Reviewed-on: https://go-review.googlesource.com/c/go/+/240597


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>