sys/benchmark: wrap BENCHMARK_FUNC in do {} while (0)
sys/benchmark: wrap BENCHMARK_FUNC in do {} while (0)

This makes the BENCHMARK_FUNC() behave more like a normal C function.

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