add OC_FilesystemView alias for compatibility

lib/filesystemview.php

+<?php
+/**
+ * Copyright (c) 2012 Robin Appelman <icewind@owncloud.com>
+ * This file is licensed under the Affero General Public License version 3 or
+ * later.
+ * See the COPYING-README file. */
+
+class OC_FilesystemView extends \OC\Files\View {}