perf stat: Fix --interval_clear option

Currently we display extra header line, like:

  # perf stat -I 1000 -a --interval-clear
  #           time             counts unit events
         insn per cycle branch-misses of all branches
       2.964917103        3855.349912      cpu-clock (msec)          #    3.855 CPUs utilized
       2.964917103             23,993      context-switches          #    0.006 M/sec
       2.964917103              1,301      cpu-migrations            #    0.329 K/sec
       ...

Fixing the condition and getting proper:

  # perf stat -I 1000 -a --interval-clear
  #           time             counts unit events
       2.359048938        1432.492228      cpu-clock (msec)          #    1.432 CPUs utilized
       2.359048938              7,613      context-switches          #    0.002 M/sec
       2.359048938                419      cpu-migrations            #    0.133 K/sec
       ...

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Fixes: 9660e08e

 ("perf stat: Add --interval-clear option")
Link: http://lkml.kernel.org/r/20180702134202.17745-2-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>