COMP: vtkMRMLSubjectHierarchyNode: Fix ignored-qualifier warning

In file included from /path/to/Slicer/Modules/Loadable/SubjectHierarchy/Widgets/qSlicerSubjectHierarchyAbstractPlugin.h:33:0,
from /path/to/Projects/Slicer/Modules/Loadable/Segmentations/qSlicerSegmentationsModuleWidget.cxx:35:
/path/to/Projects/Slicer/Libs/MRML/Core/vtkMRMLSubjectHierarchyNode.h:164:43: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
static const vtkIdType GetInvalidItemID() { return INVALID_ITEM_ID; };
^

From: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25784 3bd1e089-480b-0410-8dfb-8563597acbee