Add missing link dependency of TrackingTools/TrajectoryState on MagneticField/Engine

The missing link dependency of TrackingTools/TrajectoryState on MagneticField/Engine prevents  TrackingTools/TrajectoryState and 69 other packages dependent on it from linking.

This trivial technical pull request adds the link dependency.

The problem was introduced when an in-line function in MagneticField/Engine was rewritten to be out of line.
This fix will be queued separately to CMSSW_7_0_X, but I didn't want to wait for it to get into the ROOT6 branch.