Add empty target api:rtp_headers

The target should contain rtp_headers.{cc,h}, but downstream
dependencies must be adjusted before moving the files into the new
target.

Bug: None
Change-Id: Ie8a37c43200463762e2fdaa99d7b49d880298602
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/128570
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@{#27200}

api/BUILD.gn

@@ -63,6 +63,10 @@ rtc_static_library("create_peerconnection_factory") {
   ]
 }
 
+rtc_source_set("rtp_headers") {
+  visibility = [ "*" ]
+}
+
 rtc_static_library("libjingle_peerconnection_api") {
   visibility = [ "*" ]
   cflags = []
@@ -252,6 +256,7 @@ rtc_source_set("ortc_api") {
 
   deps = [
     ":libjingle_peerconnection_api",
+    ":rtp_headers",
     "//third_party/abseil-cpp/absl/types:optional",
   ]
 }