wmaker: Marked args as unused for compiler in X Error handlers

When an error occurs in X, the Xlib is using a callback mechanism to
execute application code to handle the problem, which means having a
fixed argument list for that application function.

It is then correct to not use all the arguments, so this patch adds the
appropriate stuff to avoid a false report from compiler.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

src/startup.c

@@ -127,6 +127,9 @@ static int catchXError(Display * dpy, XErrorEvent * error)
  */
 static int handleXIO(Display * xio_dpy)
 {
+	/* Parameter not used, but tell the compiler that it is ok */
+	(void) xio_dpy;
+
 	dpy = NULL;
 	Exit(0);
 	return 0;