perf annotate stdio: Fix column header when using --show-total-period

perf annotate stdio: Fix column header when using --show-total-period

Currently the first column header is always "Percent", fix it to show
correct column name based on given options, i.e. if using
--show-total-period, show "Event count" as a first column.

Reported-by: Milian Wolff <milian.wolff@kdab.com>
Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/c3c902e7-95bc-16d4-366f-12eb034c5c8d@gmail.com
[ Extracted from a larger patch ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>