Add availablitity macro to RTCFileVideoCapturer

Otherwise the class triggers -Wunguarded-availability clang error

Bug: webrtc:8338
Change-Id: If789c7c1af4de5b7dc24bee808b90f3835c3171d
Reviewed-on: https://webrtc-review.googlesource.com/6400
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Daniela Jovanoska Petrenko <denicija@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20142}

sdk/objc/Framework/Classes/PeerConnection/RTCFileVideoCapturer.h

@@ -17,6 +17,7 @@
  * Per design, the file capturer can only be run once and once stopped it cannot run again.
  * To run another file capture session, create new instance of the class.
  */
+NS_CLASS_AVAILABLE_IOS(10)
 @interface RTCFileVideoCapturer : RTCVideoCapturer
 
 - (void)startCapturingFromFileNamed:(NSString *)nameOfFile;