Added missing export for the flutter engine group. (#23689)

diff --git a/shell/platform/darwin/ios/framework/Headers/FlutterEngineGroup.h b/shell/platform/darwin/ios/framework/Headers/FlutterEngineGroup.h
index 82b681c..0a5cdc5 100644
--- a/shell/platform/darwin/ios/framework/Headers/FlutterEngineGroup.h
+++ b/shell/platform/darwin/ios/framework/Headers/FlutterEngineGroup.h
@@ -19,6 +19,7 @@
  * @warning This class is a work-in-progress and may change.
  * @see https://github.com/flutter/flutter/issues/72009
  */
+FLUTTER_EXPORT
 @interface FlutterEngineGroup : NSObject
 - (instancetype)init NS_UNAVAILABLE;