Make modules/audio_mixer:audio_mixer_impl publicly visible.

Follow-up CL after offline discussion with aleloi@.

Bug: None
Change-Id: I24623df7d821a44656336d5623a31cde4436c94f
No-Try: True
Reviewed-on: https://webrtc-review.googlesource.com/c/103982
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Alex Loiko <aleloi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25008}

modules/audio_mixer/BUILD.gn

@@ -16,6 +16,7 @@ group("audio_mixer") {
 }
 
 rtc_static_library("audio_mixer_impl") {
+  visibility = [ "*" ]
   sources = [
     "audio_mixer_impl.cc",
     "audio_mixer_impl.h",