perf evsel: Rename perf_evsel__group_idx() to evsel__group_idx()

As it is a 'struct evsel' method, not part of tools/lib/perf/, aka
libperf, to whom the perf_ prefix belongs.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-top.c

@@ -1051,8 +1051,7 @@ try_again:
 				goto try_again;
 			}
 
-			perf_evsel__open_strerror(counter, &opts->target,
-						  errno, msg, sizeof(msg));
+			evsel__open_strerror(counter, &opts->target, errno, msg, sizeof(msg));
 			ui__error("%s\n", msg);
 			goto out_err;
 		}