perf tools: Mark branch_info maps as referenced

As noticed by Jiri, the hist_entry->branch_info.to/from maps need to be
marked as referenced to avoid problems later on.  So we do this when the
hist_entry is allocated.

Signed-off-by: Stephane Eranian <eranian@google.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
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/20130114140245.GA4692@quad
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>