Add dummy target that clients can depend on

This will turn into a real target later.

Bug: webrtc:11967
Change-Id: I518e2ab5f2cbb19245683d9e48f2e2555fbe99d9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/187342
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32351}

pc/BUILD.gn

@@ -307,6 +307,11 @@ rtc_library("peer_connection_message_handler") {
   ]
 }
 
+# Temporary empty target to permit fixing blockers that prevent landing
+# https://webrtc-review.googlesource.com/c/src/+/186843
+rtc_source_set("rtp_parameters_conversion") {
+}
+
 rtc_library("rtp_receiver") {
   sources = [
     "rtp_receiver.cc",