Split out a separate target for SimulcastEncoderAdapter

This will allow downstream projects to use it to construct their own
injected codecs without pulling in dependencies on the software codecs.

Bug: webrtc:7925
Change-Id: If8628fedd18e57a51a8b6e5baf4f63a686bf52e8
Reviewed-on: https://webrtc-review.googlesource.com/c/107027
Reviewed-by: Anders Carlsson <andersc@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Magnus Flodman <mflodman@webrtc.org>
Commit-Queue: Jonathan Yu <yujo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#25297}
diff --git a/video/BUILD.gn b/video/BUILD.gn
index 5ac2383..08aa53e 100644
--- a/video/BUILD.gn
+++ b/video/BUILD.gn
@@ -445,6 +445,7 @@
       "../media:rtc_media",
       "../media:rtc_media_base",
       "../media:rtc_media_tests_utils",
+      "../media:rtc_simulcast_encoder_adapter",
       "../modules:module_api",
       "../modules/pacing",
       "../modules/rtp_rtcp",