perf stat: Fix metric column header display alignment

perf stat: Fix metric column header display alignment

Make the metric only display aligned.

Before:
  # perf stat --topdown -I 1000
  #           time core         cpus retiring             bad speculation      frontend bound       backend bound
       1.000394323 S0-C0           2     37.4%               12.0%               31.4%               19.2%
       1.000394323 S0-C1           2     25.1%                9.2%               43.8%               21.9%
       2.001521204 S0-C0           2     36.4%               11.4%               32.4%               19.8%
       2.001521204 S0-C1           2     26.2%                9.4%               43.1%               21.3%
       3.001930208 S0-C0           2     35.1%               10.7%               33.6%               20.6%
       3.001930208 S0-C1           2     28.9%               10.0%               40.0%               21.1%

After:
  # perf stat --topdown -I 1000
  #           time core         cpus             retiring      bad speculation       frontend bound        backend bound
       1.000303722 S0-C0           2                34.2%                 7.6%                34.2%                24.0%
       1.000303722 S0-C1           2                33.1%                 6.4%                36.9%                23.6%
       2.001281055 S0-C0           2                34.6%                 6.7%                36.8%                21.8%
       2.001281055 S0-C1           2                32.8%                 7.1%                38.1%                22.0%
       3.001546080 S0-C0           2                39.3%                 5.5%                32.7%                22.5%
       3.001546080 S0-C1           2                37.8%                 6.0%                33.1%                23.1%

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: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/20180606221513.11302-6-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>