perf diff: Add missing hists__init() call at tool start

It also uses hists/hist_entries, hists__init() should be called before
creating any evsels.

Otherwise no extra space will be allocated per perf_evsel nor this space
will be initialized when allocating a new perf_evsel instance, resulting
in reads/writes to non allocated space, oops. Fix it.

Signed-off-by: Kan Liang <kan.liang@intel.com>
Link: http://lkml.kernel.org/r/1414004561-22096-1-git-send-email-kan.liang@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>