perf tools: Prevent condition that all sort keys are elided

If given sort keys are all elided there'll be no output except for the
overhead column - actually the TUI shows a noisy output.  In this case
it'd be better to show up the sort keys rather than elide.

Before:

  $ perf report -s comm -c perf
  (...)
  # Overhead
  # ........
  #
     100.00%

After:

  $ perf report -s comm -c perf
  (...)
  # Overhead  Command
  # ........  .......
  #
     100.00%     perf

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1383900822-14609-1-git-send-email-namhyung@kernel.org


[ Us curly braces around multi-line statements, as requested by Ingo Molnar ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>