cmd/vet: add color.CMYK to the whitelist of frozen structs.

Change-Id: I5164ef9a998a0807f80668b9140eac4b2f889049
Reviewed-on: https://go-review.googlesource.com/4997
Reviewed-by: David Symonds <dsymonds@golang.org>

cmd/vet/whitelist/whitelist.go

@@ -39,6 +39,7 @@ var UnkeyedLiteral = map[string]bool{
 	// These image and image/color struct types are frozen. We will never add fields to them.
 	"image/color.Alpha16": true,
 	"image/color.Alpha":   true,
+	"image/color.CMYK":    true,
 	"image/color.Gray16":  true,
 	"image/color.Gray":    true,
 	"image/color.NRGBA64": true,