Run queue latency does not make much sense for idle `swapper` threads.

The same happens in `perf sched`:

* https://github.com/torvalds/linux/blob/v4.14/tools/perf/builtin-sched.c

```c
    /*
     * Ignore idle threads:
     */
    if (!strcmp(thread__comm_str(work_list->thread), "swapper"))
        return;
```

```c
static bool is_idle_sample(struct perf_sample *sample,
	       struct perf_evsel *evsel)
{
    /* pid 0 == swapper == idle task */
    if (strcmp(perf_evsel__name(evsel), "sched:sched_switch") == 0)
        return perf_evsel__intval(evsel, sample, "prev_pid") == 0;

    return sample->pid == 0;
}
```