GN: Add audio_conference_mixer_unittests to modules_unittests.

Was removed by accident in https://codereview.webrtc.org/2629923002/

BUG=webrtc:6954
R=kjellander@webrtc.org
NOTRY=True

Review-Url: https://codereview.webrtc.org/2641203002
Cr-Original-Commit-Position: refs/heads/master@{#16170}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 9410b51037ab6b2497b03f3888a3e1defd9dcbeb
2 files changed