COMP: ImageRegistrationHistogramPlotter HistogramWriter visibility.

This fixes:

ImageRegistrationHistogramPlotter.cxx:171:7 warning: 'HistogramWriter'
declared with greater visibility than the type of its fild
'HistogramWriter::m_HistogramFileWriter'

Change-Id: I52936d5b35bfc2b30a2c7b21bb9b862406f3bba7
Merge topic 'HistogramWriter-visibility2'

2a3100c COMP: ImageRegistrationHistogramPlotter HistogramWriter visibility.