perf diff: Fix -o/--order option behavior (again)

Commit 21e6d842 ("perf diff: Use perf_hpp__register_sort_field
interface") changed list_add() to perf_hpp__register_sort_field().

This resulted in a behavior change since the field was added to the tail
instead of the head.  So the -o option is mostly ignored due to its
order in the list.

This patch fixes it by adding perf_hpp__prepend_sort_field().

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Fixes: 21e6d842 ("perf diff: Use perf_hpp__register_sort_field interface")
Link: http://lkml.kernel.org/r/20170118051457.30946-2-namhyung@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>