Fix build with -no-opengl and on watchOS in general

This fixes a regression introduced in 8e70241dcc.

Task-number: QTBUG-63631
Change-Id: I5a2c23e06b790a482e1542ac2db3dcf25927caf2
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/gui/kernel/qwindow.cpp

@@ -2634,10 +2634,12 @@ QWindow *QWindowPrivate::topLevelWindow() const
     return window;
 }
 
+#if QT_CONFIG(opengl)
 QOpenGLContext *QWindowPrivate::shareContext() const
 {
     return qt_gl_global_share_context();
 };
+#endif
 
 /*!
     Creates a local representation of a window created by another process or by

src/gui/kernel/qwindow_p.h

@@ -130,7 +130,9 @@ public:
 
     QWindow *topLevelWindow() const;
 
+#if QT_CONFIG(opengl)
     virtual QOpenGLContext *shareContext() const;
+#endif
 
     virtual QWindow *eventReceiver() { Q_Q(QWindow); return q; }