tests/bench_xtimer: fix typo 'emtpy' -> 'empty'
tests/bench_xtimer: fix typo 'emtpy' -> 'empty'

OTAkeys/RIOT,jasonatran/RIOT,authmillenon/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kYc0o/RIOT,jasonatran/RIOT,miri64/RIOT,kYc0o/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,OlegHahm/RIOT,miri64/RIOT,ant9000/RIOT,kaspar030/RIOT,kYc0o/RIOT,miri64/RIOT,authmillenon/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kYc0o/RIOT,kaspar030/RIOT,authmillenon/RIOT,miri64/RIOT,miri64/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,ant9000/RIOT,kaspar030/RIOT,authmillenon/RIOT,OlegHahm/RIOT,jasonatran/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,kYc0o/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,ant9000/RIOT