[QNN EP] Exclude QnnGpu.dll from Windows x64 NuGet (#24487)
### Description
Excludes QnnGpu.dll from Windows x64 NuGet package because it is not
available for that architecture.
### Motivation and Context
Fix failure in QNN packaging pipeline:
```shell
CreateNativePackage:
Generating nuspec for the native Microsoft.ML.OnnxRuntime.QNN nuget package...
python ..\tools\nuget\generate_nuspec_for_native_nuget.py --package_version 1.22.0-dev-20250421-0439-2abab8d --package_name Microsoft.ML.OnnxRuntime.QNN --target_architecture x64 --build_config RelWithDebInfo --native_build_path D:\a\_work\1\b\RelWithDebInfo\RelWithDebInfo --packages_path D:\a\_work\1\b\packages --ort_build_path D:\a\_work\1\b --sources_path D:\a\_work\1\s --commit_id 2abab8d39e08819d0e44833f9afee0a37c9aaa75 --is_release_build False --execution_provider None --nuspec_name NativeNuget.nuspec
1 file(s) copied.
1 file(s) copied.
nuspec_name: NativeNuget.nuspec
Bundling native shared library artifacts into Microsoft.ML.OnnxRuntime nuget package...
nuget pack NativeNuget.nuspec
Attempting to build package from 'NativeNuget.nuspec'.
##[error]EXEC(0,0): Error NU5019: File not found: 'D:\a\_work\1\b\RelWithDebInfo\RelWithDebInfo\QnnGpu.dll'.
EXEC : error NU5019: File not found: 'D:\a\_work\1\b\RelWithDebInfo\RelWithDebInfo\QnnGpu.dll'. [D:\a\_work\1\s\csharp\OnnxRuntime.CSharp.proj]
##[error]csharp\OnnxRuntime.CSharp.proj(109,5): Error MSB3073: The command "nuget pack NativeNuget.nuspec" exited with code 1.
```
Introduced by this PR:
https://github.com/microsoft/onnxruntime/pull/24435
