Move rtc_error.{h,cc} to its own build target.

Bug: webrtc:8733
Change-Id: Idd34d9a88ae62a01b9ea50719872f8188069211e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/152320
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29148}
diff --git a/p2p/BUILD.gn b/p2p/BUILD.gn
index 1583cfb..6a9438d 100644
--- a/p2p/BUILD.gn
+++ b/p2p/BUILD.gn
@@ -100,6 +100,7 @@
     ":stun_types",
     "../api:libjingle_peerconnection_api",
     "../api:packet_socket_factory",
+    "../api:rtc_error",
     "../api:scoped_refptr",
     "../api/rtc_event_log",
     "../api/transport:enums",