fix: Header filename mismatch causing ffigen to produce empty output

[BrutalCoding]

Description

Some minor context how I got here:

Been fighting with ffigen for about a week now to get a project called `llama_cpp` ported over to Dart as a convenient package. I've got ffi working in a side project of mine (shady.ai) but creating a plugin_ffi package has been troublesome.

Before this PR, upon checking out the repository, things seemed to work at first glance. No compile errors. I was able to execute the test and run commands with success in the native_add_library package.

However, I was unable to re-generate Dart code from the native header (specified in ffigen.yaml), turns out it's due to a typo in the [headers] section.

Steps I had taken prior this PR:

1. Navigate to dart_lang_native/pkgs/native_assets_cli/example/native_add_library
2. Execute dart --enable-experiment=native-assets test
3. Test passed.
4. Happy me
5. Added new C code
6. Ran ffigen tooling
7. Analyzer will show some syntax errors here and there due to the missing missing references
8. native_add_library.dart seems to be an empty file now aside from the file comments.

Before

Result of lib/native_add_library.dart:

// Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

// AUTO GENERATED FILE, DO NOT EDIT.
//
// Generated by `package:ffigen`.
// ignore_for_file: type=lint

After

Now, with this PR, the file (lib/native_add_library.dart) is looking good again.

// Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

// AUTO GENERATED FILE, DO NOT EDIT.
//
// Generated by `package:ffigen`.
// ignore_for_file: type=lint
import 'dart:ffi' as ffi;

@ffi.Native<ffi.Int32 Function(ffi.Int32, ffi.Int32)>(symbol: 'add')
external int add(
  int a,
  int b,
);

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[dcharkes]

Thanks @BrutalCoding! 🚀

I must have missed this when moving the examples around.

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