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

Linking external ObjC / Swift apps fails when the app code is using any
of the new frame-buffer classes RTCI420Buffer, RTCMutableI420Buffer, or
RTCCVPixelBuffer. To fix, we need to add the appropriate attribute to
the classes (e.g. using the RTC_EXPORT macro).

BUG=None

Review-Url: https://codereview.webrtc.org/2961293002
Cr-Commit-Position: refs/heads/master@{#18840}

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