Make header files self contained.

This CL adds #includes to header files in order to make them
self contained after the preprocessor pass.

Bug: b/251890128
Change-Id: I81c3ba38fb8ab8a2bbd151ba99aa871fae9f1b1b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/278422
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38327}
diff --git a/api/video_codecs/bitstream_parser.h b/api/video_codecs/bitstream_parser.h
index 0d8d014..86ce192 100644
--- a/api/video_codecs/bitstream_parser.h
+++ b/api/video_codecs/bitstream_parser.h
@@ -10,9 +10,11 @@
 
 #ifndef API_VIDEO_CODECS_BITSTREAM_PARSER_H_
 #define API_VIDEO_CODECS_BITSTREAM_PARSER_H_
+
 #include <stddef.h>
 #include <stdint.h>
 
+#include "absl/types/optional.h"
 #include "api/array_view.h"
 
 namespace webrtc {