Generator should warn if unused interfaces are part of bindings

An interface without `[Protocol]` or `[BaseType]` will be ignored when generating binding code.

### Steps to Reproduce

1. Add the following in `appkit.cs`

```csharp
interface NSAccessibilityOutline2 : NSAccessibilityTable {}
```

2. Build

### Expected Behavior

`NSAccessibilityOutline2` is not part of the API but there were no warnings.

### Actual Behavior

No warnings. Incorrect bindings might result.

### Environment

master