Fix Linux compilation error.
Fix Linux compilation error.

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