Fixed a compilation error with the newest dev panel API.

plugins/thunar-tpa/thunar-tpa.c

@@ -257,7 +257,7 @@ thunar_tpa_size_changed (XfcePanelPlugin *panel_plugin,
   gtk_widget_set_size_request (GTK_WIDGET (panel_plugin), size, size);
 
 #if LIBXFCE4PANEL_CHECK_VERSION (4,13,0)
-  image_size = xfce_panel_plugin_get_icon_size (panel_plugin, GTK_WIDGET (plugin->button));
+  image_size = xfce_panel_plugin_get_icon_size (panel_plugin);
 #else
   image_size = size - 2; // fall-back for older panel versions
 #endif