Reimplement QGraphicsEffect::sourceChanged() properly

Reimplement QGraphicsEffect::sourceChanged() properly

sourceChanged() takes one argument, not zero.

tests/auto/widgets/graphicsview/qgraphicseffectsource/tst_qgraphicseffectsource.cpp:125:
warning: 'CustomEffect::sourceChanged' hides overloaded virtual function
[-Woverloaded-virtual]
    void sourceChanged()
         ^
src/widgets/effects/qgraphicseffect.h:104:
hidden overloaded virtual function 'QGraphicsEffect::sourceChanged'
declared here
    virtual void sourceChanged(ChangeFlags flags);
                 ^

Change-Id: If227b7b79c4a2fc3d21f93e371510ddccb408d7d
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@nokia.com>