macOS: Fix unused variable in window:willUseFullScreenContentSize:

Change-Id: I419f884f4145dbe2b60751bf6cde3968cf34fe4a
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/cocoa/qnswindowdelegate.mm

@@ -88,6 +88,7 @@
 */
 - (NSSize)window:(NSWindow *)window willUseFullScreenContentSize:(NSSize)proposedSize
 {
+    Q_UNUSED(window);
     Q_ASSERT(NSEqualSizes(m_cocoaWindow->screen()->geometry().size().toCGSize(), proposedSize));
     return proposedSize;
 }