Cocoa: Remove compilation warning

Change-Id: Ib530e7ef4643de07aec91c4af0de8bbedef9882a
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>

src/plugins/platforms/cocoa/qcocoaapplicationdelegate.mm

@@ -113,6 +113,7 @@ static void cleanupCocoaApplicationDelegate()
 
 - (void)updateScreens:(NSNotification *)notification
 {
+    Q_UNUSED(notification);
     if (QCocoaIntegration *ci = dynamic_cast<QCocoaIntegration *>(QGuiApplicationPrivate::platformIntegration()))
         ci->updateScreens();
 }