perf evlist: Fix set event list leader

perf evlist: Fix set event list leader

The __perf_evlist__set_leader() was setting the leader for all events in
the list except the first. Which means it assumed the first event
already had event->leader = event.

Seems like this should be the role of the function to also do this. This
is a requirement for an upcoming patch set.

Signed-off-by: Stephane Eranian <eranian@google.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Tested-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20130131125437.GA3656@quad
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>