Support the platform iOS for Flutter (#1079)

flutter/sherpa_onnx/lib/sherpa_onnx.dart

@@ -19,11 +19,9 @@ String? _path;
 
 // see also
 // https://github.com/flutter/codelabs/blob/main/ffigen_codelab/step_05/lib/ffigen_app.dart
+// https://api.flutter.dev/flutter/dart-io/Platform-class.html
 final DynamicLibrary _dylib = () {
-  if (Platform.isIOS) {
-    throw UnsupportedError('Unknown platform: ${Platform.operatingSystem}');
-  }
-  if (Platform.isMacOS) {
+  if (Platform.isMacOS || Platform.isIOS) {
     if (_path == null) {
       return DynamicLibrary.open('libsherpa-onnx-c-api.dylib');
     } else {