fix(ios): 修复逐帧请求头编译失败

ios/Runner/AppDelegate.swift

@@ -2,6 +2,8 @@ import Flutter
 import UIKit
 import AVFoundation
 
+private let thumbnailHTTPHeaderFieldsKey = "AVURLAssetHTTPHeaderFieldsKey"
+
 private final class ThumbnailResponseGate {
   private let lock = NSLock()
   private var completed = false
@@ -104,7 +106,7 @@ final class RunnerFlutterViewController: FlutterViewController {
 
       let options: [String: Any]? = headers.isEmpty
         ? nil
-        : [AVURLAssetHTTPHeaderFieldsKey: headers]
+        : [thumbnailHTTPHeaderFieldsKey: headers]
       let asset = AVURLAsset(url: assetUrl, options: options)
       let generator = AVAssetImageGenerator(asset: asset)
       generator.appliesPreferredTrackTransform = true