Export QQuickWheelEvent privately

It's mainly intended for QML use cases: both MouseArea and WheelHandler
have wheel(QQuickWheelEvent *) signals. But the QML compiler generates
C++, and other use cases are possible. For the signals to be useful in
C++, their arguments need to be exported. Besides, the other classes in
this file are exported the same way (for example
21b3b74c99e2ee8dedb711ba448066eb308979dc; some were made private in
83e820ba76).

QQuickWheelEvent was added in f2e1be963f

Pick-to: 6.5
Fixes: QTBUG-111230
Change-Id: I448aa633c7e7deb2e5f12e29bcb1c449be00a63d
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/quick/items/qquickevents_p_p.h

@@ -172,7 +172,7 @@ private:
 };
 
 #if QT_CONFIG(wheelevent)
-class QQuickWheelEvent : public QObject
+class Q_QUICK_PRIVATE_EXPORT QQuickWheelEvent : public QObject
 {
     Q_OBJECT
     Q_PROPERTY(const QPointingDevice *device READ pointingDevice CONSTANT)