Added vtkPolyLine.h header to example_supervoxels.cpp

In 1.7.0 the supervoxel example doesn't build with Visual Studio 2012 (and others: https://github.com/PointCloudLibrary/pcl/issues/202).
Merge pull request #206 from henningpohl/patch-1

Added vtkPolyLine.h header to example_supervoxels.cpp