[cleanup] Remove useless includes.

Manual cleanup guided by include-what-you-use diagnostic.

Bug: webrtc:8311
Change-Id: I00be03392cc7ee005101427ea7dc701621ccea68
Reviewed-on: https://webrtc-review.googlesource.com/c/103320
Commit-Queue: Yves Gerey <yvesg@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25013}
diff --git a/modules/rtp_rtcp/source/contributing_sources.h b/modules/rtp_rtcp/source/contributing_sources.h
index 1a4a572..b6201ce 100644
--- a/modules/rtp_rtcp/source/contributing_sources.h
+++ b/modules/rtp_rtcp/source/contributing_sources.h
@@ -18,7 +18,8 @@
 
 #include "absl/types/optional.h"
 #include "api/array_view.h"
-#include "api/rtpreceiverinterface.h"
+#include "api/rtpreceiverinterface.h"  // For RtpSource
+#include "rtc_base/timeutils.h"        // For kNumMillisecsPerSec
 
 namespace webrtc {