WPrefs: Marked args as unused for compiler in event callback code

The WINGs toolkit dispatch events on widgets using callbacks, which means
having a fixed argument list for the handling 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>

WPrefs.app/editmenu.c

@@ -574,6 +574,9 @@ static void closeMenuAction(WMWidget * w, void *data)
 {
 	WEditMenu *menu = (WEditMenu *) data;
 
+	/* Parameter not used, but tell the compiler that it is ok */
+	(void) w;
+
 	WMAddIdleHandler(WMDestroyWidget, menu->closeB);
 	menu->closeB = NULL;