Fix AVFoundation framework import

When building the WebRTC project for iOS, the build will fail on Xcode 9
because of a missing framework-header (AVFoundation). This pull-request
will add the missing "#import <AVFoundation/AVFoundation.h>" line to the
"RTCCameraVideoCapturer" class.

BUG=webrtc:7846

Review-Url: https://codereview.webrtc.org/2944753002
Cr-Commit-Position: refs/heads/master@{#18698}

AUTHORS

@@ -51,6 +51,7 @@ Vladimir Beloborodov <VladimirTechMan@gmail.com>
 Vicken Simonian <vsimon@gmail.com>
 Victor Costan <costan@gmail.com>
 Xiaohong Xu <freemine@yeah.net>
+Hans Knoechel <hans@hans-knoechel.de>
 
 &yet LLC <*@andyet.com>
 Agora IO <*@agora.io>

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

@@ -9,6 +9,7 @@
  */
 
 #import <Foundation/Foundation.h>
+#import <AVFoundation/AVFoundation.h>
 
 #import <WebRTC/RTCMacros.h>
 #import <WebRTC/RTCVideoCapturer.h>