cppia can't link extern abstract enum

Trying to use one from cppia results in an error like:
```
> ./host/Host script.cppia
in Script::0x7fffd27d7cb0
GetFieldByName at :0 main
Could not link static _Main.E_Impl_::N1 (3)
Error : Bad link
```

**n.hpp**
```
#pragma once

enum N {
N1 = 7,
N2 = -1
};
```

**Host.hx**
```
@:enum @:unreflective extern abstract E (EImpl) {
@:native("N1") var E1;
@:native("N2") var E2;
}

@:include("./n.hpp") @:unreflective @:native("N") extern class EImpl {}

class Host {
public static function main () {
trace(E.E1);
trace(E.E2);
}
}
```

**Script.hx**
```
class Script {
public static function main () {
Host.main();
trace(Host.E.E1);
trace(Host.E.E2);
}
}
```

Build the host `haxe -main cpp.cppia.Host -cpp host Host -D scriptable`, the script `haxe -main Script -cppia script.cppia` and run `./host/Host script.cppia`.
Using `-jit` doesn't change anything.

It can't access `_Main.E_Impl_::N1` since it's not in the host.

Sorry for the multi file example, couldn't figure out how to reduce to a single file.