Add empty build target modules/video_coding:video_coding_legacy

A followup cl will move VideoCodingModule and related code into this
target.

Bug: webrtc:7408
Change-Id: Iade572b597769456c9b8c76f584500e2bd9a58f0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/152280
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29122}

modules/video_coding/BUILD.gn

@@ -209,6 +209,10 @@ rtc_source_set("video_codec_interface") {
   ]
 }
 
+rtc_source_set("video_coding_legacy") {
+  visibility = [ ":video_coding_unittests" ]
+}
+
 rtc_source_set("codec_globals_headers") {
   visibility = [ "*" ]
   sources = [