add import

Author: Chris Yang

shell/platform/darwin/ios/framework/Source/FlutterTextInputPlugin.mm

@@ -7,6 +7,8 @@
 #import <Foundation/Foundation.h>
 #import <UIKit/UIKit.h>
 
+#include "unicode/uchar.h"
+
 #include "flutter/fml/logging.h"
 #include "flutter/fml/platform/darwin/string_range_sanitization.h"