cmd/webp-manual-test: add a "build ignore" tag, so that "go install golang.org/x/image/..." doesn't install the manual testing program.
cmd/webp-manual-test: add a "build ignore" tag, so that "go install
golang.org/x/image/..." doesn't install the manual testing program.

Also add a missing copyright notice.

Change-Id: I7ab0a438df09ac8882e7d2482d2bf96d4260f8eb
Reviewed-on: https://go-review.googlesource.com/9277
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

golang/image