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/Preferences.c

@@ -62,6 +62,9 @@ static void borderCallback(WMWidget * w, void *data)
 	char buffer[64];
 	int i;
 
+	/* Parameter not used, but tell the compiler that it is ok */
+	(void) w;
+
 	i = WMGetSliderValue(panel->borderS);
 
 	if (i == 0)