Address dynamic class property deprecation warnings

templates/PhotosIndex.php

@@ -10,6 +10,7 @@ class PhotosIndex extends SubTemplate
 {
 	protected $mosaic;
 	protected $show_edit_buttons;
+	protected $show_headers;
 	protected $show_labels;
 	protected $row_limit = 1000;
 	protected $previous_header = '';