COMP: Fix warning, unused unexported symbol in ctkPixmapIconEngine

ctkPixmapIconEngine is the temporary public version of QPixmapIconEngine.
It is a light version compared to Qt.

Libs/Widgets/ctkPixmapIconEngine.h

@@ -106,7 +106,6 @@ private:
     ctkPixmapIconEngineEntry *tryMatch(const QSize &size, QIcon::Mode mode, QIcon::State state);
     QVector<ctkPixmapIconEngineEntry> pixmaps;
 
-    friend QDataStream &operator<<(QDataStream &s, const QIcon &icon);
     friend class QIconThemeEngine;
 };