[Cleanup] Add missing #include. Remove useless ones. IWYU part 2.

This is a follow-up to
https://webrtc-review.googlesource.com/c/src/+/106280.
This time the whole code base is covered.
Some files may have not been fixed though, whenever the IWYU tool
was breaking the build.

Bug: webrtc:8311
Change-Id: I2c31f552a87e887d33931d46e87b6208b1e483ef
Reviewed-on: https://webrtc-review.googlesource.com/c/111965
Commit-Queue: Yves Gerey <yvesg@google.com>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25830}
diff --git a/pc/dtlssrtptransport.h b/pc/dtlssrtptransport.h
index cac560e..995e537 100644
--- a/pc/dtlssrtptransport.h
+++ b/pc/dtlssrtptransport.h
@@ -11,13 +11,16 @@
 #ifndef PC_DTLSSRTPTRANSPORT_H_
 #define PC_DTLSSRTPTRANSPORT_H_
 
-#include <memory>
-#include <string>
 #include <vector>
 
+#include "absl/types/optional.h"
+#include "api/cryptoparams.h"
+#include "api/rtcerror.h"
 #include "p2p/base/dtlstransportinternal.h"
+#include "p2p/base/packettransportinternal.h"
 #include "pc/srtptransport.h"
 #include "rtc_base/buffer.h"
+#include "rtc_base/third_party/sigslot/sigslot.h"
 
 namespace webrtc {