Fix format-security warning in error.c

gtk_message_dialog_new() takes a format string which should be "%s" in
this case, not the string to display. Reported by Sebastian.

src/error.c

@@ -14,6 +14,7 @@ void show_error(GtkWindow *w, char *s) {
     GTK_DIALOG_DESTROY_WITH_PARENT | GTK_DIALOG_MODAL,
     GTK_MESSAGE_ERROR,
     GTK_BUTTONS_CLOSE,
+    "%s",
     s
   );
   gtk_widget_show(dialog);