govet: suppress warning about Parent field that repeated json tag "parent"

```
image/image.go:65:2: structtag: struct field Parent repeats json tag "parent" also at image.go:39 (govet)
	Parent     ID        `json:"parent,omitempty"`
```

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

image/image.go

@@ -62,7 +62,7 @@ type V1Image struct {
 // Image stores the image configuration
 type Image struct {
 	V1Image
-	Parent     ID        `json:"parent,omitempty"`
+	Parent     ID        `json:"parent,omitempty"` //nolint:govet
 	RootFS     *RootFS   `json:"rootfs,omitempty"`
 	History    []History `json:"history,omitempty"`
 	OSVersion  string    `json:"os.version,omitempty"`