perf: Missing field in PERF_RECORD_SAMPLE documentation

While trying to write a perf_event/mmap test for my perf_event
test-suite I came across a missing field description in the
PERF_RECORD_SAMPLE documentation in perf_event.h

Signed-off-by: Vince Weaver <vincent.weaver@maine.edu>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/alpine.DEB.2.02.1301081439300.24507@vincent-weaver-1.um.maine.edu
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>