perf trace: Do not show the runtime_ms for a thread when not collecting it

perf trace: Do not show the runtime_ms for a thread when not collecting it

That field is only updated when we use the "sched:sched_stat_runtime"
tracepoint, and that is only done so far when we use the '--stat' command line
option, without it we get just zeros, confusing the users:

Without this patch:

  # trace -a -s sleep 1
  <SNIP>
   qemu-system-x86 (9931), 468 events, 9.6%, 0.000 msec

     syscall     calls    total       min       avg       max      stddev
                          (msec)    (msec)    (msec)    (msec)        (%)
     ---------- ------ --------- --------- --------- ---------     ------
     ppoll          98   982.374     0.000    10.024    29.983     12.65%
     write          34     0.401     0.005     0.012     0.027      5.49%
     ioctl         102     0.347     0.002     0.003     0.007      3.08%

   firefox (10871), 1856 events, 38.2%, 0.000 msec

                          (msec)    (msec)    (msec)    (msec)        (%)
     ---------- ------ --------- --------- --------- ---------     ------
     poll          395   934.873     0.000     2.367    17.120     11.51%
     recvmsg       395     0.988     0.001     0.003     0.021      4.20%
     read          106     0.460     0.002     0.004     0.007      3.17%
     futex          24     0.108     0.001     0.004     0.010     10.05%
     mmap            2     0.041     0.016     0.021     0.026     23.92%
     write           6     0.027     0.004     0.004     0.005      2.52%

After this patch that ', 0.000 msecs' gets suppressed when --stat is not
in use.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-p7emqrsw7900tdkg43v9l1e1@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>