libperf event: Ensure tracing data is multiple of 8 sized

Perf's synthetic-events.c will ensure 8-byte alignment of tracing
data, writing it after a perf_record_header_tracing_data event. Add
padding to struct perf_record_header_tracing_data to make it 16-byte
rather than 12-byte sized.

tools/lib/perf/include/perf/event.h

@@ -291,6 +291,7 @@ struct perf_record_header_event_type {
 struct perf_record_header_tracing_data {
 	struct perf_event_header header;
 	__u32			 size;
+	__u32			 pad;
 };
 
 #define PERF_RECORD_MISC_BUILD_ID_SIZE (1 << 15)