Fix qquickframebufferobject.h syncqt warning

QtQuick: WARNING: qquickframebuffe robject.h includes QQuickItem when
it should include QtQuick/QQuickItem

Change-Id: Ia702bab8881ade601693cbee495b1dc6a5e14abb
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>