perf: Update ABI comment

For some mysterious reason the sample_id field of PERF_RECORD_MMAP went AWOL.

Reported-by: Vince Weaver <vince@deater.net>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 40a1fb8..7f6d584 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -528,6 +528,7 @@
 	 *	u64				len;
 	 *	u64				pgoff;
 	 *	char				filename[];
+	 * 	struct sample_id		sample_id;
 	 * };
 	 */
 	PERF_RECORD_MMAP			= 1,