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

The mechanism of Notifications in the WINGs toolkit is relying on
callbacks to dispatch notifications, 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/MouseSettings.c

@@ -151,6 +151,9 @@ static void returnPressed(void *observerData, WMNotification * notification)
 {
 	_Panel *panel = (_Panel *) observerData;
 
+	/* Parameter not used, but tell the compiler that it is ok */
+	(void) notification;
+
 	speedChange(NULL, panel);
 }