Reland "Move api/rtp_headers.h to its own build target."

This is a reland of a67050debcb5a3461a452a7928d7aaea1562747e

Original change's description:
> Move api/rtp_headers.h to its own build target.
>
> Reduces dependencies on the libjingle_peerconnection_api target from
> lower-level code.
>
> Bug: None
> Change-Id: I98576fc718c396cc0f720c3770acd2b696b9df89
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/128565
> Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Commit-Queue: Niels Moller <nisse@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#27213}

Bug: None
Tbr: kwiberg@webrtc.org
Change-Id: If15b05957e50bb8f18a33c2ed1321e672311b626
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/127895
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27216}
diff --git a/audio/BUILD.gn b/audio/BUILD.gn
index 82c6482..fc42002 100644
--- a/audio/BUILD.gn
+++ b/audio/BUILD.gn
@@ -41,6 +41,7 @@
     "../api:array_view",
     "../api:call_api",
     "../api:libjingle_peerconnection_api",
+    "../api:rtp_headers",
     "../api:scoped_refptr",
     "../api:transport_api",
     "../api/audio:aec3_factory",