[feature/compiled-dic] Add HttpKernel; the Extension class is loaded from it

The phpbb_di_extension_* classes extend the Extension class that is included
in the HttpKernel component. If I don't include HttpKernel I have to implement
the ExtensionInterface from the DependencyInjection component and have to
define all of the methods; this way, I can just overwrite what I need.

PHPBB3-10864

phpBB/composer.json

@@ -4,6 +4,7 @@
 		"symfony/config": "2.1.*",
 		"symfony/dependency-injection": "2.1.*",
 		"symfony/event-dispatcher": "2.1.*",
+		"symfony/http-kernel": "2.1.*",
 		"symfony/yaml": "2.1.*"
 	},
 	"require-dev": {