Fix deprecation of QMouseEvent::source() and flags()

Amends 53496e67f0b78645c6080e9218c7a36bc5a9d76d:
- move flags() out of the QT_DEPRECATED_SINCE block in the header
- add QT_DEPRECATED_SINCE block around source() implementation

Change-Id: Id52fa6b04a13efbede3e6ac440060f90b283e773
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit ba8f9d8d92ba4fd9aaf6e0885b56b56e25acfc45)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/kernel/qevent.cpp

@@ -803,10 +803,12 @@ QMouseEvent::~QMouseEvent()
     \sa Qt::MouseEventSource
     \sa QGraphicsSceneMouseEvent::source()
 */
+#if QT_DEPRECATED_SINCE(6, 0)
 Qt::MouseEventSource QMouseEvent::source() const
 {
     return Qt::MouseEventSource(m_source);
 }
+#endif
 
 /*!
     \since 5.3

src/gui/kernel/qevent.h

@@ -272,8 +272,8 @@ public:
     QT_DEPRECATED_VERSION_X_6_0("Use globalPosition()")
     QPointF screenPos() const { return globalPosition(); }
     Qt::MouseEventSource source() const;
-    Qt::MouseEventFlags flags() const;
 #endif // QT_DEPRECATED_SINCE(6, 0)
+    Qt::MouseEventFlags flags() const;
 };
 
 class Q_GUI_EXPORT QHoverEvent : public QSinglePointEvent