Add empty build target modules/video_coding:video_coding_legacy
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 <d2c43c210eae6feef04f53bae50885e8152edcca@webrtc.org>
Commit-Queue: Niels Moller <0057859e47d61d90529d0f66d9ef692cefedb46a@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#29122}

TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc