perf hists: Introduce hists__link_hierarchy()

The hists__link_hierarchy() is to support hierarchy reports with an
event group.  When it matches the leader event and the other members
(using hists__match_hierarchy()), it also needs to link unmatched member
entries with a dummy leader event so that it can show up in the output.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20160913074552.13284-3-namhyung@kernel.org


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