Mark public ObjC APIs as explicitly visible.

webrtc/api/objc/RTCRtpSender.h

 /*
  *  Copyright 2016 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/api/objc/RTCMediaStreamTrack.h"
 #import "webrtc/api/objc/RTCRtpParameters.h"
+#import "webrtc/base/objc/RTCMacros.h"
 
 NS_ASSUME_NONNULL_BEGIN
 
+RTC_EXPORT
 @protocol RTCRtpSender <NSObject>
 
 /** The currently active RTCRtpParameters, as defined in
  *  https://www.w3.org/TR/webrtc/#idl-def-RTCRtpParameters.
  */
 @property(nonatomic, readonly) RTCRtpParameters *parameters;
 
 /** The RTCMediaStreamTrack associated with the sender.
  *  Note: reading this property returns a new instance of
  *  RTCMediaStreamTrack. Use isEqual: instead of == to compare
  *  RTCMediaStreamTrack instances.
  */
 @property(nonatomic, readonly) RTCMediaStreamTrack *track;
 
 /** Set the new RTCRtpParameters to be used by the sender.
  *  Returns YES if the new parameters were applied, NO otherwise.
  */
 - (BOOL)setParameters:(RTCRtpParameters *)parameters;
 
 @end
 
+RTC_EXPORT
 @interface RTCRtpSender : NSObject <RTCRtpSender>
 
 - (instancetype)init NS_UNAVAILABLE;
 
 @end
 
 NS_ASSUME_NONNULL_END