COMP: ImageRegistrationHistogramPlotter Clone never referenced.

For:

  /scratch/dashboards/Linux-x86_64-icc13/ITK/Examples/Registration/ImageRegistrationHistogramPlotter.cxx(394):
  warning #177: function "<unnamed>::CommandIterationUpdate::Clone" was
  declared but never referenced

Change-Id: I1d53d34b221132c76b1638e00e1ce9d5be47f730
Merge topic 'histogram-icc'

d67d2223 COMP: ImageRegistrationHistogramPlotter Clone never referenced.