[cpp] @:include for extern enum abstract not working

In the following example, the generated code in `Main.cpp` does not have `#include "mode.hpp"`, causing native compilation error.

```haxe
@:include("mode.hpp") @:native("foo::Mode")
extern enum abstract Mode(Int) {
final X;
}
```

```haxe
class Main {
static function main() {
trace(Mode.X);
}
```