Revert "Removing unneeded dependency."

This reverts commit 06f66c72600e58438ba9caf9f523e00a519ef3c0.

Reason for revert: Breaks downstream project.

Original change's description:
> Removing unneeded dependency.
> 
> The //audio build target does not depend on the
> builtin_audio_encoder_factory, this CL removes it from the dependency
> list in order to avoid to propagate symbols that are not supposed to
> be there.
> 
> Bug: webrtc:9528
> Change-Id: Ib3868ee93f61057f61283faaa83e0633ebfdea90
> Reviewed-on: https://webrtc-review.googlesource.com/89002
> Reviewed-by: Yves Gerey <yvesg@webrtc.org>
> Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
> Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#24019}

TBR=mbonadei@webrtc.org,ossu@webrtc.org,yvesg@webrtc.org

Change-Id: Icf8f0ad4e7f5cce96fa1c0491a281ef2fd2e713f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:9528
Reviewed-on: https://webrtc-review.googlesource.com/89400
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24023}

audio/BUILD.gn

@@ -54,6 +54,7 @@ rtc_static_library("audio") {
     "../api/audio:audio_frame_api",
     "../api/audio:audio_mixer_api",
     "../api/audio_codecs:audio_codecs_api",
+    "../api/audio_codecs:builtin_audio_encoder_factory",
     "../call:bitrate_allocator",
     "../call:call_interfaces",
     "../call:rtp_interfaces",