QCocoaWindow - fix a flag test

NSBorderlessWindowMask is just 0, not the 0-th bit, so testing
styleMask & NSBorderlessWindowMask never works, use
original flags instead.

Change-Id: I47da7b5b7795fce88dec88c948a68197fa428991
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>

src/plugins/platforms/cocoa/qcocoawindow.mm

@@ -877,7 +877,7 @@ void QCocoaWindow::setWindowFlags(Qt::WindowFlags flags)
         m_inSetStyleMask = false;
         [m_nsWindow setLevel:level];
         setWindowShadow(flags);
-        if (!(styleMask & NSBorderlessWindowMask)) {
+        if (!(flags & Qt::FramelessWindowHint)) {
             setWindowTitle(window()->title());
         }