Solve internal class usage (#1767)

Author: VincentLanglet

src/DependencyInjection/SonataPageExtension.php

@@ -20,10 +20,10 @@
 use Symfony\Component\Config\FileLocator;
 use Symfony\Component\DependencyInjection\ContainerBuilder;
 use Symfony\Component\DependencyInjection\Definition;
+use Symfony\Component\DependencyInjection\Extension\Extension;
 use Symfony\Component\DependencyInjection\Extension\PrependExtensionInterface;
 use Symfony\Component\DependencyInjection\Loader\PhpFileLoader;
 use Symfony\Component\DependencyInjection\Reference;
-use Symfony\Component\HttpKernel\DependencyInjection\Extension;
 
 /**
  * @author Thomas Rabaix <[email protected]>