Fix warning on 64 bit Windows build.
Fix warning on 64 bit Windows build.

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