time_bench_sample: also time overhead of for-loop

Timing at the nanosec level, we need to know the overhead
introduced by the for loop itself.

CPU E5-2695:
Type:for_loop Per elem: 0 cycles(tsc) 0.359 ns (step:0)
 - (measurement period time:0.359792753 sec time_interval:359792753)
 - (invoke count:1000000000 tsc_interval:899480653)

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>