WUtil: Marked args as unused for compiler in WUtil's API code

There is a function in WUtil's API that take a parameter for consistency
reason, but actually does not need the argument.

As it is a case we know about, this patch adds the appropriate stuff to
tell the compiler we are ok with this to avoid a false report.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

WINGs/bagtree.c

@@ -758,5 +758,8 @@ void *WMBagIteratorAtIndex(WMBag * self, int index, WMBagIterator * ptr)
 
 int WMBagIndexForIterator(WMBag * bag, WMBagIterator ptr)
 {
+	/* Parameter not used, but tell the compiler that it is ok */
+	(void) bag;
+
 	return ((W_Node *) ptr)->index;
 }