perf evlist: Fix splice_list_tail() not setting evlist

Commit d49e4695

 ("perf evsel: Add a backpointer to the evlist a
evsel is in") updated perf_evlist__add() but not
perf_evlist__splice_list_tail().

This illustrates that it is better if perf_evlist__splice_list_tail()
calls perf_evlist__add() instead of duplicating the logic, so do that.
This will also simplify a subsequent fix for propagating maps.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@intel.com>
Link: http://lkml.kernel.org/r/1441699142-18905-6-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>