Include of non-modular header inside framework module

I always get this issue when I run an iOS project after adding flutter_appauth dependency:

```
Lexical or Preprocessor Issue (Xcode): Include of non-modular header inside framework module 'flutter_appauth.AppAuthIOSAuthorization': '/Users/emad/StudioProjects/d-ticket/ios/Pods/Headers/Public/AppAuth/AppAuth.h'
/Users/emad/.pub-cache/hosted/pub.dev/flutter_appauth-6.0.4/ios/Classes/AppAuthIOSAuthorization.h:0:8

Lexical or Preprocessor Issue (Xcode): Include of non-modular header inside framework module 'flutter_appauth.OIDExternalUserAgentIOSNoSSO': '/Users/emad/StudioProjects/d-ticket/ios/Pods/Headers/Public/AppAuth/AppAuth.h'
/Users/emad/.pub-cache/hosted/pub.dev/flutter_appauth-6.0.4/ios/Classes/OIDExternalUserAgentIOSNoSSO.h:19:8

Lexical or Preprocessor Issue (Xcode): Include of non-modular header inside framework module 'flutter_appauth.FlutterAppAuth': '/Users/emad/StudioProjects/d-ticket/ios/Pods/Headers/Public/AppAuth/AppAuth.h'
/Users/emad/.pub-cache/hosted/pub.dev/flutter_appauth-6.0.4/ios/Classes/FlutterAppAuth.h:6:8

Lexical or Preprocessor Issue (Xcode): Include of non-modular header inside framework module 'flutter_appauth.FlutterAppauthPlugin': '/Users/emad/StudioProjects/d-ticket/ios/Pods/Headers/Public/AppAuth/AppAuth.h'
/Users/emad/.pub-cache/hosted/pub.dev/flutter_appauth-6.0.4/ios/Classes/FlutterAppauthPlugin.h:10:8

Parse Issue (Xcode): Could not build module 'flutter_appauth'
/Users/emad/StudioProjects/d-ticket/ios/Runner/GeneratedPluginRegistrant.m:33:8
```
flutter_appauth version ==> 6.0.4, also with previous versions I got this issue.
I've tried to upgrade Flutter to the latest version but with no success.
I've tried cleaning the project and building it again but with no success.

Start with ios/Classes/AppAuthIOSAuthorization.h, OIDExternalUserAgentIOSNoSSO.h, FlutterAppAuth.h, and FlutterAppauthPlugin.h, then inspect the referenced Pods/Headers/Public/AppAuth/AppAuth.h and Runner/GeneratedPluginRegistrant.m errors. Reproduce the iOS build and determine the project or dependency change needed so the flutter_appauth module builds without non-modular-header errors.

Written by the indexing model from the issue text.