Add atomic to a counter of AlignSiModule

http://goetz.web.cern.ch/goetz/mutable_2019-05-11.html#InnerDetector_Other

 nightly/master/2019-05-11T2151:InnerDetector/InDetAlignEvent/InDetAlignTrkInfo/InDetAlignTrkInfo/AlignSiModule.h +67


    67	  float m_length;
    68	  float m_width[3];
    69	  mutable int m_nhits;
    70	  bool m_dead;
    71	  bool m_stereo;