Add missing file to p2p.

Bug: None
Change-Id: Ic0c183fb63ba2fdcd07044b7063e96928150884b
Reviewed-on: https://webrtc-review.googlesource.com/37681
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21511}

p2p/BUILD.gn

@@ -65,6 +65,7 @@ rtc_static_library("rtc_p2p") {
     "base/tcpport.cc",
     "base/tcpport.h",
     "base/transport.h",
+    "base/transportchannelimpl.h",
     "base/transportdescription.cc",
     "base/transportdescription.h",
     "base/transportdescriptionfactory.cc",