Tracking4D: resolve compilation error from previous cherry-pick: replace...

Tracking4D: resolve compilation error from previous cherry-pick: replace refTrack (non-existent) by track

src/modules/Tracking4D/Tracking4D.cpp

@@ -257,7 +257,7 @@ StatusCode Tracking4D::run(std::shared_ptr<Clipboard> clipboard) {
                               (distanceY * distanceY) / (spatial_cuts_[det].y() * spatial_cuts_[det].y());
 
                 // If this is the second cluster of the track, keep it for now
-                if(refTrack->nClusters() == 1 && ne == 0) {
+                if(track->nClusters() == 1 && ne == 0) {
                     closestClusterDistance = distance;
                     closestCluster = newCluster;
                 } else {