tink_json cannot stringify cpp.CastCharStar

```hx
@:include('./MessagePayload.h')
@:structAccess
@:jsonStringify(function (mp:MessagePayload) return { someFloat: mp.someFloat, cStr: mp.cStr })
extern class MessagePayload {
	var someFloat: cpp.Float32;
	var cStr: CastCharStar;
}
```

and error

     MessagePayload.hx:6: characters 82-96 : tink_json cannot stringify cpp.CastCharStar

