perf trace: Fix summary percentage when processing files

Getting a divide by 0 when events are processed from a file:

   perf trace -i perf.data -s
   ...
   dnsmasq (1684), 10 events, inf%, 0.000 msec

The problem is that the event count is not incremented as events are
processed. With this patch:

   perf trace -i perf.data -s
   ...
   dnsmasq (1684), 10 events, 8.9%, 0.000 msec

Signed-off-by: David Ahern <dsahern@gmail.com>
Link: http://lkml.kernel.org/r/1386211302-31303-4-git-send-email-dsahern@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>