perf hists: Introduce output_resort_cb method

When dealing with nested hist entries it's helpful to have a way to
resort those nested objects.

Adding optional callback call into output_resort function and following
new interface function:

  typedef int (*hists__resort_cb_t)(struct hist_entry *he);

  void hists__output_resort_cb(struct hists *hists,
                               struct ui_progress *prog,
                               hists__resort_cb_t cb);

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1470074555-24889-7-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>