perf tools: Do not auto-remove Children column if --fields given

Depending on the configuration perf inserts/removes the Children
column in the output automatically.  But it might not be what user
wants if [s]he give --fields option explicitly.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Rodrigo Campos <rodrigo@sdfg.com.ar>
Cc: Arun Sharma <asharma@fb.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/1401335910-16832-18-git-send-email-namhyung@kernel.org


Signed-off-by: Jiri Olsa <jolsa@kernel.org>