perf timechart: Get number of CPUs from perf header

perf timechart: Get number of CPUs from perf header

Print all CPUs, even if there were no events (use perf header to get
number of CPUs).

This is required to support topology in the next patch.

Signed-off-by: Stanislav Fomichev <stfomichev@yandex-team.ru>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ramkumar Ramachandra <artagnon@gmail.com>
Link: http://lkml.kernel.org/r/1385995056-20158-4-git-send-email-stfomichev@yandex-team.ru
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>