Update IWYU tool with a mapping file

Also apply IWYU to all .cc files in pc/, and correct BUILD file to match.
Note: Some files came out wrong when iwyu was applied. These are not included.

Bug: none
Change-Id: Ib5ea46b8fcc505414d0447cca7218ad3afc2e321
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/252280
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36064}
diff --git a/pc/rtp_sender.cc b/pc/rtp_sender.cc
index 110b5aa..dc53105 100644
--- a/pc/rtp_sender.cc
+++ b/pc/rtp_sender.cc
@@ -12,6 +12,7 @@
 
 #include <algorithm>
 #include <atomic>
+#include <string>
 #include <utility>
 #include <vector>