QGuiApplication: remove Windows-only variable from non-Windows builds

...lest some code tries to set it to no possible effect.

Change-Id: I7aed05200c1abeda1d2f9d88ceb99a5ce6132dbf
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/gui/kernel/qguiapplication_p.h

@@ -331,7 +331,9 @@ private:
 
     static QGuiApplicationPrivate *self;
     static int m_fakeMouseSourcePointId;
+#ifdef Q_OS_WIN
     QSharedPointer<QColorTrcLut> m_a8ColorProfile;
+#endif
     QSharedPointer<QColorTrcLut> m_a32ColorProfile;
 
     bool ownGlobalShareContext;