Properly export the symbols of video frame-buffer classes for link-time

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCVideoFrameBuffer.h

Index: webrtc/sdk/objc/Framework/Headers/WebRTC/RTCVideoFrameBuffer.h
diff --git a/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCVideoFrameBuffer.h b/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCVideoFrameBuffer.h
index 59986b8da36de109bd153bad9380d6dc267a1e77..63505ab2de12bcba0b2b725899dc94e7b8a846aa 100644
--- a/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCVideoFrameBuffer.h
+++ b/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCVideoFrameBuffer.h
@@ -65,6 +65,7 @@ RTC_EXPORT
 @end
 
 /** RTCVideoFrameBuffer containing a CVPixelBufferRef */
+RTC_EXPORT
 @interface RTCCVPixelBuffer : NSObject <RTCVideoFrameBuffer>
 
 @property(nonatomic, readonly) CVPixelBufferRef pixelBuffer;
@@ -89,10 +90,12 @@ RTC_EXPORT
 @end
 
 /** RTCI420Buffer implements the RTCI420Buffer protocol */
+RTC_EXPORT
 @interface RTCI420Buffer : NSObject <RTCI420Buffer>
 @end
 
 /** Mutable version of RTCI420Buffer */
+RTC_EXPORT
 @interface RTCMutableI420Buffer : RTCI420Buffer <RTCMutableI420Buffer>
 @end