Solve internal class usage (#968)

Author: VincentLanglet

src/DependencyInjection/SonataClassificationExtension.php

@@ -18,8 +18,8 @@
 use Symfony\Component\Config\Definition\Processor;
 use Symfony\Component\Config\FileLocator;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
+use Symfony\Component\DependencyInjection\Extension\Extension;
 use Symfony\Component\DependencyInjection\Loader\PhpFileLoader;
-use Symfony\Component\HttpKernel\DependencyInjection\Extension;
 
 /**
  * SonataClassificationBundleExtension.