perf: Update ABI comment

For some mysterious reason the sample_id field of PERF_RECORD_MMAP went AWOL.
Reported-by: NVince Weaver <vince@deater.net>
Signed-off-by: NPeter Zijlstra <peterz@infradead.org>
Signed-off-by: NIngo Molnar <mingo@kernel.org>

include/uapi/linux/perf_event.h

@@ -528,6 +528,7 @@ enum perf_event_type {
 	 *	u64				len;
 	 *	u64				pgoff;
 	 *	char				filename[];
+	 * 	struct sample_id		sample_id;
 	 * };
 	 */
 	PERF_RECORD_MMAP			= 1,