wmaker: Marked args as unused for compiler in XExposeEvent callback code

The custom drawing code for windows is handled using callback functions,
which means having a fixed argument list for that 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/menu.c

@@ -1667,6 +1667,9 @@ void wMenuScroll(WMenu * menu, XEvent * event)
 
 static void menuExpose(WObjDescriptor * desc, XEvent * event)
 {
+	/* Parameter not used, but tell the compiler that it is ok */
+	(void) event;
+
 	wMenuPaint(desc->parent);
 }