Add placeholder target to move rtc_error out of the main API target.

No-Try: True
Bug: webrtc:8733
Change-Id: Ia9a3e2155b87d908b783d1ee2ba9aa7067083354
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/152284
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29127}

api/BUILD.gn

@@ -204,6 +204,12 @@ rtc_static_library("libjingle_peerconnection_api") {
   ]
 }
 
+rtc_source_set("rtc_error") {
+  visibility = [ "*" ]
+  # Placeholder target for the migration of rtc_error.{h,cc} out of
+  # libjingle_peerconnection_api.
+}
+
 rtc_source_set("packet_socket_factory") {
   visibility = [ "*" ]
   sources = [