Move evsel::idx to perf_evsel::idx, so we can move the group interface
to libperf.

Committer notes:

Fixup evsel->idx usage in tools/perf/util/bpf_counter_cgroup.c, that
appeared in my tree in my local tree.

Also fixed up these:

$ find tools/perf/ -name "*.[ch]" | xargs grep 'evsel->idx'
tools/perf/ui/gtk/annotate.c:                      evsel->idx + i);
tools/perf/ui/gtk/annotate.c:                   evsel->idx);
$

That running 'make -C tools/perf build-test' caught.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Requested-by: Shunsuke Nakamura <nakamura.shun@fujitsu.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20210706151704.73662-3-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>