Fix macOS benchmarks

[rozbb]

I get an error when running sudo python3 main_count_cycles_mac_m1.py:

Traceback (most recent call last):
  File "projects/ProjectTempo/main_count_cycles_mac_m1.py", line 473, in <module>
    main()
    ~~~~^^
  File "projects/ProjectTempo/main_count_cycles_mac_m1.py", line 434, in main
    avg, rsd = bench_gen_matrix(gen_matrix, k)
               ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "projects/ProjectTempo/main_count_cycles_mac_m1.py", line 308, in bench_gen_matrix
    start = read_cycles()
  File "projects/ProjectTempo/main_count_cycles_mac_m1.py", line 102, in read_cycles
    fixed = read_fixed()
  File "projects/ProjectTempo/main_count_cycles_mac_m1.py", line 96, in read_fixed
    raise OSError("kpc_get_thread_counters failed")
OSError: kpc_get_thread_counters failed

I think this is because the benchmark is missing a kperf init line. This PR adds that line and fixes the benches on my MBP Air M1.

[afonsoarriaga]

Thank you, @rozbb! I must have removed that line during a cleanup, and it went unnoticed since I don't reboot my laptop very often.