minor fix: add missing source file to s3benchmark CMakeLists
minor fix: add missing source file to s3benchmark CMakeLists

cvmfs/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,DrDaveD/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,cvmfs/cvmfs,DrDaveD/cvmfs