app: fix CLang build.

Our Weekly CLang build for Debian failed with:

> ../app/widgets/gimpfiledialog.c:924:37: error: label at end of compound statement: expected statement
>                             default:

That is not a problem locally (with GCC), but I guess we can just add a
g_return_if_reached() in here as this code path is not supposed to ever
be reached indeed.

app/widgets/gimpfiledialog.c

@@ -922,6 +922,11 @@ gimp_file_dialog_proc_changed (GimpFileProcView *view,
                               ext = gimp_plug_in_procedure_get_save_extensions (proc, TRUE);
                               break;
                             default:
+                              /* Should not happen, it's an
+                               * GTK_FILE_CHOOSER_ACTION_SAVE file
+                               * chooser.
+                               */
+                              g_return_if_reached ();
                             }
 
                           g_string_append (s, ext);