Do not use HttpKernel Extension when not needed for 2.7

Author: Tobion

Tests/Functional/Bundle/TestBundle/DependencyInjection/TestExtension.php

@@ -12,7 +12,7 @@
 namespace Symfony\Bundle\FrameworkBundle\Tests\Functional\Bundle\TestBundle\DependencyInjection;
 
 use Symfony\Component\DependencyInjection\ContainerBuilder;
-use Symfony\Component\HttpKernel\DependencyInjection\Extension;
+use Symfony\Component\DependencyInjection\Extension\Extension;
 use Symfony\Component\DependencyInjection\Extension\PrependExtensionInterface;
 
 class TestExtension extends Extension implements PrependExtensionInterface