Move VideoStreamEncoderInterface to api/.

Bug: webrtc:8830
Change-Id: I17908b4ef6a043acf22e2110b9672012d5fa7fc0
Reviewed-on: https://webrtc-review.googlesource.com/74481
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23334}
diff --git a/video/BUILD.gn b/video/BUILD.gn
index 25ec808..3782217 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -64,6 +64,7 @@
     "../api:transport_api",
     "../api/video:video_frame",
     "../api/video:video_frame_i420",
+    "../api/video:video_stream_encoder",
     "../api/video_codecs:video_codecs_api",
     "../call:bitrate_allocator",
     "../call:call_interfaces",