Add empty build target p2p:stun_types

Preparation for cl
https://webrtc-review.googlesource.com/c/src/+/150945.

Bug: webrtc:8733
Change-Id: I98ed03a9117792f372d9c0fb5bc073879b4a18dc
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/151122
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29033}

p2p/BUILD.gn

@@ -15,6 +15,11 @@ group("p2p") {
   ]
 }
 
+# TODO(bugs.webrtc.org/8733): Move base/stun.{h,cc} into this target,
+# after downstream dependencies are updated.
+rtc_source_set("stun_types") {
+}
+
 rtc_static_library("rtc_p2p") {
   visibility = [ "*" ]
   sources = [