perf report: Add an indication of what time slices are used

Add a time slices indication to the perf report header.

For example,

  # perf report --stdio --time 10%

  # Total Lost Samples: 0
  #
  # Samples: 9K of event 'cycles:ppp' (time slices: 10%)
  # Event count (approx.): 8951288803

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>
Reviewed-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1515596433-24653-6-git-send-email-yao.jin@linux.intel.com


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