Add missing include to FlutterThreadSynchronizer (flutter#38337)

Author: knopp

shell/platform/darwin/macos/framework/Source/FlutterThreadSynchronizer.mm

@@ -2,7 +2,9 @@
 #import "fml/synchronization/waitable_event.h"
 
 #import <QuartzCore/QuartzCore.h>
+
 #include <mutex>
+#include <vector>
 
 @interface FlutterThreadSynchronizer () {
   std::mutex _mutex;