perf annotate: Show group event string for stdio

When we enable the group, for tui/stdio2, the output first line includes
the group event string. While for stdio, it will show only one event.

For example,

perf record -e cycles,branches ./div
perf annotate --group --stdio

    Percent |      Source code & Disassembly of div for cycles (44407 samples)
    ......

The first line doesn't include the event 'branches'.

With this patch, it will show the correct group even string.

perf annotate --group --stdio

    Percent |      Source code & Disassembly of div for cycles, branches (44407 samples)
    ......
Signed-off-by: NJin Yao <yao.jin@linux.intel.com>
Suggested-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1526989115-14435-1-git-send-email-yao.jin@linux.intel.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/annotate.c

@@ -1965,6 +1965,7 @@ int symbol__annotate_printf(struct symbol *sym, struct map *map,
 	u64 len;
 	int width = symbol_conf.show_total_period ? 12 : 8;
 	int graph_dotted_len;
+	char buf[512];
 
 	filename = strdup(dso->long_name);
 	if (!filename)
@@ -1977,8 +1978,11 @@ int symbol__annotate_printf(struct symbol *sym, struct map *map,
 
 	len = symbol__size(sym);
 
-	if (perf_evsel__is_group_event(evsel))
+	if (perf_evsel__is_group_event(evsel)) {
 		width *= evsel->nr_members;
+		perf_evsel__group_desc(evsel, buf, sizeof(buf));
+		evsel_name = buf;
+	}
 
 	graph_dotted_len = printf(" %-*.*s|	Source code & Disassembly of %s for %s (%" PRIu64 " samples)\n",
 				  width, width, symbol_conf.show_total_period ? "Period" :