trace_processor: add import on stddef.h in stats.h

Change-Id: I778d4ca8ff01cf211bc00c57d468ac61206c10c4

src/trace_processor/stats.h

diff --git a/src/trace_processor/stats.h b/src/trace_processor/stats.h
index 1684b09..2fb4dc0 100644
--- a/src/trace_processor/stats.h
+++ b/src/trace_processor/stats.h
@@ -17,6 +17,8 @@
 #ifndef SRC_TRACE_PROCESSOR_STATS_H_
 #define SRC_TRACE_PROCESSOR_STATS_H_
 
+#include <stddef.h>
+
 namespace perfetto {
 namespace trace_processor {
 namespace stats {