perf stat: Fix BPF program section name

It seems the recent libbpf got more strict about the section name.
I'm seeing a failure like this:

  $ sudo ./perf stat -a --bpf-counters --for-each-cgroup ^. sleep 1
  libbpf: prog 'on_cgrp_switch': missing BPF prog type, check ELF section name 'perf_events'
  libbpf: prog 'on_cgrp_switch': failed to load: -22
  libbpf: failed to load object 'bperf_cgroup_bpf'
  libbpf: failed to load BPF skeleton 'bperf_cgroup_bpf': -22
  Failed to load cgroup skeleton

The section name should be 'perf_event' (without the trailing 's').
Although it's related to the libbpf change, it'd be better fix the
section name in the first place.

Fixes: 944138f0 ("perf stat: Enable BPF counter with --for-each-cgroup")
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: bpf@vger.kernel.org
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/r/20220916184132.1161506-2-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/bpf_skel/bperf_cgroup.bpf.c

@@ -176,7 +176,7 @@ static int bperf_cgroup_count(void)
 }
 
 // This will be attached to cgroup-switches event for each cpu
-SEC("perf_events")
+SEC("perf_event")
 int BPF_PROG(on_cgrp_switch)
 {
 	return bperf_cgroup_count();