perf hists: Export a couple of hist functions

perf hists: Export a couple of hist functions

These are necessary for multi threaded sample processing:

 - hists__get__get_rotate_entries_in()
 - hists__collapse_insert_entry()
 - __hists__init()

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Noel Grandin <noelgrandin@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1452158050-28061-14-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>