Mark built-in software video codecs as poisonous.

The goal is to make these injectable, and only VP8 and VP9 specific
targets should depend on them.

Bug: webrtc:7925
Change-Id: Ie9239a54d197fe70c93de0582797211fef6997a2
Reviewed-on: https://webrtc-review.googlesource.com/72082
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23021}
diff --git a/BUILD.gn b/BUILD.gn
index 99e2006..c1ccb5c 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -597,3 +597,6 @@
 #
 group("poison_audio_codecs") {
 }
+
+group("poison_software_video_codecs") {
+}