QIconLoader: don't inherit QObject

QIconLoader did not use the services from QObject and the
Q_OBJECT macro was missing, too, so external code couldn't
have used it in a qobject_cast or inherits(), either.

Change-Id: I1f33dd540fa2ded48d871d848a77eee743a4e3c0
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/gui/image/qiconloader_p.h

@@ -158,7 +158,7 @@ private:
     bool m_valid;
 };
 
-class Q_GUI_EXPORT QIconLoader : public QObject
+class Q_GUI_EXPORT QIconLoader
 {
 public:
     QIconLoader();