perf stat: Add units to nanosec-based counters

perf stat: Add units to nanosec-based counters

Ingo pointed out that the task-clock counter should have the units
explicitly stated since it is not a counter.

Before:

perf stat -a -- sleep 1

 Performance counter stats for 'sleep 1':

      16186.874834 task-clock          #   16.154 CPUs utilized
...

After:

perf stat -a -- sleep 1

 Performance counter stats for 'system wide':

      16146.402138 task-clock (msec)   #   16.125 CPUs utilized
...

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1380400080-9211-4-git-send-email-dsahern@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>