Fix Bencher warning in CI

Per https://github.com/dholroyd/mpeg2ts-reader/pull/58#issuecomment-2088030066

> WARNING: The following Measures do not have a Threshold. Without a Threshold, no Alerts will ever be generated!
> 
>  -  Instructions (instructions)
>  -  L1 Accesses (l1-accesses)
>  -  L2 Accesses (l2-accesses)
>  -  RAM Accesses (ram-accesses)
>  -  Total Accesses (total-accesses)