COMP: MultiVolumeExplorer - Anticipate logic headers reorganization.

Soon, as a compilation speedup, the base class "vtkMRMLAbstractLogic"
will forward declared classes in its definition file.

This commit fix compilation issue that would have occurred following
integration of the topic "finetjul/Slicer@vtkmrmlabstractlogic-clean-includes-trunk"

Fixes #3494

Co-authored-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

From: Andrey Fedorov <fedorov@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22693 3bd1e089-480b-0410-8dfb-8563597acbee