Add a declaration to QIODevice

It isn't getting #included nor forward-declared at this point. That was
only an indirect inclusion.

qglsceneformatplugin.h:64:5: error: ‘QIODevice’ does not name a type

Change-Id: Ie118c1c0de0c206077215b22fe435d3e6a57fa73
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>