The code assumes (correctly) that perf_event_header's size is
non-zero. Write that out so static analysis also knows it.

Signed-off-by: Petri Latvala <petri.latvala@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>