Fix undeclared use of QSGFlatColorMaterial when QT_NO_OPENGL is defined

QtWebEngine fails to build with -no-opengl due to

  src/core/delegated_frame_node.cpp

trying to use QSGFlatColorMaterial without including the header.

Change-Id: I98c12d3c482f7cacb2f61dd13ae32ee0a5740ae2
Reviewed-by: Michael Brüning <michael.bruning@qt.io>