api/test: Fix visibility of MockAudioSink

Bug: webrtc:9620
Change-Id: Iaff79457b37937f0d0dc6734ea34ede61aff883d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/226326
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Commit-Queue: Florent Castelli <orphis@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34495}
diff --git a/api/BUILD.gn b/api/BUILD.gn
index fa92790..767056f 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -835,6 +835,7 @@
   }
 
   rtc_source_set("mock_audio_sink") {
+    visibility = [ "*" ]
     testonly = true
     sources = [ "test/mock_audio_sink.h" ]