perf_event: Update PERF_EVENT_FORK header definition

PERF_EVENT_FORK always outputs the time field, so update the header
to reflect this.

Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <20090922123424.GD19453@kryten>
Signed-off-by: Ingo Molnar <mingo@elte.hu>