BUILD.gn file for api/audio.

Targets containing files in api/audio are moved from api/BUILD.gn to
api/audio/BUILD.gn.

Bug: webrtc:8844
Change-Id: Ib7ea4b7eb3c2ea38ef8261a1fc5c2b4674985981
Reviewed-on: https://webrtc-review.googlesource.com/54360
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Gustaf Ullberg <gustaf@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22074}
diff --git a/audio/BUILD.gn b/audio/BUILD.gn
index 5b5c723..cfd320a 100644
--- a/audio/BUILD.gn
+++ b/audio/BUILD.gn
@@ -47,11 +47,11 @@
   deps = [
     "..:webrtc_common",
     "../api:array_view",
-    "../api:audio_mixer_api",
     "../api:call_api",
     "../api:libjingle_peerconnection_api",
     "../api:optional",
     "../api:transport_api",
+    "../api/audio:audio_mixer_api",
     "../api/audio_codecs:audio_codecs_api",
     "../api/audio_codecs:builtin_audio_encoder_factory",
     "../call:bitrate_allocator",