Adding in objc vars for WebRTC GN config.
Adding in objc vars for WebRTC GN config.

Appears to be needed after https://codereview.chromium.org/1941053002.

R=ivoc@webrtc.org
TBR=kjellander@webrtc.org

Review URL: https://codereview.webrtc.org/1941413002 .

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#12609}

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