Add missing virtual destructor.
Add missing virtual destructor.

scamille/simbody,chrisdembia/simbody,scamille/simbody,elkingtonmcb/simbody,chrisdembia/simbody,simbody/simbody,robojukie/simbody,elkingtonmcb/simbody,Gjacquenot/simbody,Gjacquenot/simbody,chrisdembia/simbody,robojukie/simbody,scamille/simbody,elen4/simbody,Gjacquenot/simbody,elen4/simbody,elkingtonmcb/simbody,simbody/simbody,robojukie/simbody