Help IWYU understand headers.

Change-Id: Ieaa9e70d566b759d9c3664381bc5a6ab48c2575f
diff --git a/aos/flatbuffers.h b/aos/flatbuffers.h
index 5da7466..39b072b 100644
--- a/aos/flatbuffers.h
+++ b/aos/flatbuffers.h
@@ -7,7 +7,7 @@
 #include "absl/types/span.h"
 #include "aos/containers/resizeable_buffer.h"
 #include "aos/macros.h"
-#include "flatbuffers/flatbuffers.h"
+#include "flatbuffers/flatbuffers.h"  // IWYU pragma: export
 #include "glog/logging.h"
 
 namespace aos {