[Cleanup] Add missing #include. Remove useless ones.

This CL is the result of running include-what-you-use tool on part
of the code base (audio target and dependencies) plus manual fixes.

bug: webrtc:8311
Change-Id: I277d281ce943c3ecc1bd45fd8d83055931743604
Reviewed-on: https://webrtc-review.googlesource.com/c/106280
Commit-Queue: Yves Gerey <yvesg@google.com>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25311}
diff --git a/rtc_base/stringencode.h b/rtc_base/stringencode.h
index 5d436df..09bf77f 100644
--- a/rtc_base/stringencode.h
+++ b/rtc_base/stringencode.h
@@ -11,9 +11,12 @@
 #ifndef RTC_BASE_STRINGENCODE_H_
 #define RTC_BASE_STRINGENCODE_H_
 
+#include <stddef.h>
 #include <string>
+#include <type_traits>
 #include <vector>
 
+#include "absl/types/optional.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/string_to_number.h"