Mark public ObjC APIs as explicitly visible.

BUG=

Review URL: https://codereview.webrtc.org/1899573002

Cr-Commit-Position: refs/heads/master@{#12431}
diff --git a/webrtc/api/objc/RTCAudioTrack.h b/webrtc/api/objc/RTCAudioTrack.h
index b5ea649..0f4b69d 100644
--- a/webrtc/api/objc/RTCAudioTrack.h
+++ b/webrtc/api/objc/RTCAudioTrack.h
@@ -9,9 +9,11 @@
  */
 
 #import "webrtc/api/objc/RTCMediaStreamTrack.h"
+#import "webrtc/base/objc/RTCMacros.h"
 
 NS_ASSUME_NONNULL_BEGIN
 
+RTC_EXPORT
 @interface RTCAudioTrack : RTCMediaStreamTrack
 
 - (instancetype)init NS_UNAVAILABLE;