GN: Add targets for AppRTCDemo on Mac+iOS

webrtc/examples/objc/AppRTCDemo/mac/main.m

 /*
  *  Copyright 2014 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#import <Cocoa/Cocoa.h>
+#import <AppKit/AppKit.h>
 
 #import "APPRTCAppDelegate.h"
 
 int main(int argc, char* argv[]) {
   @autoreleasepool {
     [NSApplication sharedApplication];
     APPRTCAppDelegate* delegate = [[APPRTCAppDelegate alloc] init];
     [NSApp setDelegate:delegate];
     [NSApp run];
   }
 }