QEventPoint: forward-declare QMutableEventPoint before befriending it

Otherwise the friend declaration declares the name, which here is ok,
but can cause duplicate names in other situations.

Pick-to: 6.3 6.2
Change-Id: I695f9c5c116b90ba52e1b99e15f31e732e63d357
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/gui/kernel/qeventpoint.h

@@ -50,6 +50,7 @@ QT_BEGIN_NAMESPACE
 
 class QEventPointPrivate;
 QT_DECLARE_QESDP_SPECIALIZATION_DTOR_WITH_EXPORT(QEventPointPrivate, Q_GUI_EXPORT)
+class QMutableEventPoint;
 
 class Q_GUI_EXPORT QEventPoint
 {