perf diff: Use perf_hpp__register_sort_field interface

Using perf_hpp__register_sort_field interface instead of directly adding
the entry.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-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/1452158050-28061-13-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-diff.c

@@ -1207,7 +1207,7 @@ static int ui_init(void)
 		BUG_ON(1);
 	}
 
-	list_add(&fmt->sort_list, &perf_hpp__sort_list);
+	perf_hpp__register_sort_field(fmt);
 	return 0;
 }