perf tools: Add ->sort() member to struct sort_entry

Currently, what the sort_entry does is just identifying hist entries
so that they can be grouped properly.  However, with -F option
support, it indeed needs to sort entries appropriately to be shown to
users.  So add ->sort() member to do it.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Link: http://lkml.kernel.org/r/1400480762-22852-13-git-send-email-namhyung@kernel.org


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