perf stat: Declare some references static

This references are not exported, use static declaration.

Signed-off-by: Robert Richter <robert.richter@amd.com>
Cc: Ingo Molnar <mingo@kernel.org>
Link: http://lkml.kernel.org/r/1333643188-26895-4-git-send-email-robert.richter@amd.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>