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/TexturePanel.c

@@ -524,6 +524,9 @@ static void gradDeleteCallback(WMWidget * w, void *data)
 
 static void colorWellObserver(void *self, WMNotification * n)
 {
+	/* Parameter not used, but tell the compiler that it is ok */
+	(void) n;
+
 	updateSGradButtons(self);
 }