Add atomic to a mutable counter of iPatTrackFollower.

http://goetz.web.cern.ch/goetz/mutable_2019-06-10.html#Other_iPat

nightly/master/2019-06-10T2153:Reconstruction/iPat/iPatTrackFollower/iPatTrackFollower/TrackBuilder.h +52

    50	
    51	    // counters
    52	    mutable int			m_buildCount;
    53	
    54	};