Add final missing header files before enabling orphans check.

Bug: None
Change-Id: I4fdac7481ac73b1e6035802530c834e273fb3cdc
Reviewed-on: https://webrtc-review.googlesource.com/38200
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Patrik Höglund <phoglund@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21531}
diff --git a/api/BUILD.gn b/api/BUILD.gn
index d8e7cd0..6ac235b 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -48,6 +48,7 @@
     "mediaconstraintsinterface.cc",
     "mediaconstraintsinterface.h",
     "mediastreaminterface.cc",
+    "mediastreaminterface.h",
     "mediastreamproxy.h",
     "mediastreamtrackproxy.h",
     "mediatypes.cc",