Use correct closure namespace in Extendable class

This fixes the error
<blockquote>Argument 1 passed to October\Rain\Extension\Extendable::extend() must be an instance of October\Rain\Extension\Closure, instance of Closure given</blockquote>
when extending Extendables.

Example code to replicate:
```php
BackendController::extend(function($controller) {
    $controller->addJs(__DIR__.'/foobar.js');
});
```