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: Jin Yao <yao.jin@linux.intel.com>
Suggested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Arnaldo 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.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>