Lambda function missing parens in generated D import file

```D
enum i = (x => x * 2)(3);
```

Converting the above file to a D import file with `dmd -H -c test.d` results in file test.di:
```D
// D import file generated from 'test.d'
enum i = (x) => x * 2(3);
```

The result is missing parens around the lambda function, so it is not the same, because the call expression tries to call a number.
