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}
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