COMP: Fix "inconsistent-missing-override" warnings

[hina-shah]

This commit adds the VTK_OVERRIDE macros to fix warnings like the one
reported below occurring when building OpenIGTLink against VTK 8:

In file included from /path/to/Slicer-build/Slicer-build/Modules/Remote/OpenIGTLinkIF/MRML/vtkIGTLToMRMLPointsPython.cxx:12:
In file included from /path/to/Slicer-build/OpenIGTLinkIF/MRML/vtkIGTLToMRMLPoints.h:19:
/path/to/Slicer-build/OpenIGTLinkIF/MRML/vtkIGTLToMRMLBase.h:58:8: warning: 'PrintSelf' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
  void PrintSelf(ostream& os, vtkIndent indent);
       ^
/path/to/Slicer-build/VTKv8/Common/Core/vtkObject.h:115:8: note: overridden virtual function is here
  void PrintSelf(ostream& os, vtkIndent indent) VTK_OVERRIDE;
       ^

[jcfr]

Thanks for the patch 👍

Closing. Integrated in 0914dee