Fix including extensions for actualize_script

Extensions could be included multiple times. It resulted in an error.

lib/Minz/ExtensionManager.php

@@ -114,7 +114,7 @@ class Minz_ExtensionManager {
 		$entry_point_filename = $info['path'] . '/' . self::$ext_entry_point;
 		$ext_class_name = $info['entrypoint'] . 'Extension';
 
-		include($entry_point_filename);
+		include_once($entry_point_filename);
 
 		// Test if the given extension class exists.
 		if (!class_exists($ext_class_name)) {