perf/x86: Fix missing struct before structure name

When CONFIG_PERF_EVENTS disabled, there will have a compiliation
error, because missing struct before structure name.

Signed-off-by: Jovi Zhang <bookjovi@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Jiri Kosina <trivial@kernel.org>
Link: http://lkml.kernel.org/r/CACV3sbKF%3DCX%2B2jWEWesfCA6rBoQ3wDM4-5ac9MuBtVbCtMRHdQ@mail.gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>