Mark public ObjC APIs as explicitly visible.

webrtc/api/objc/RTCVideoSource.h

 /*
  *  Copyright 2015 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #import <Foundation/Foundation.h>
 
+#import "webrtc/base/objc/RTCMacros.h"
+
 typedef NS_ENUM(NSInteger, RTCSourceState) {
   RTCSourceStateInitializing,
   RTCSourceStateLive,
   RTCSourceStateEnded,
   RTCSourceStateMuted,
 };
 
 NS_ASSUME_NONNULL_BEGIN
 
+RTC_EXPORT
 @interface RTCVideoSource : NSObject
 
 /** The current state of the RTCVideoSource. */
 @property(nonatomic, readonly) RTCSourceState state;
 
 - (instancetype)init NS_UNAVAILABLE;
 
 @end
 
 NS_ASSUME_NONNULL_END