add missing var

lib/private/files/objectstore/objectstorestorage.php

@@ -38,6 +38,10 @@ class ObjectStoreStorage extends \OC\Files\Storage\Common {
 	 * @var \OCP\Files\ObjectStore\IObjectStore $objectStore
 	 */
 	protected $objectStore;
+	/**
+	 * @var string $id
+	 */
+	protected $id;
 	/**
 	 * @var \OC\User\User $user
 	 */