fix: add missing files provider to NVIDIA distribution (#3479)

# What does this PR do?
The rag-runtime tool requires files API as a dependency, but the NVIDIA
distribution was missing the files provider configuration. Thus, when
running:

```
llama stack build --distro nvidia --image-type venv
```
And then:
```
llama stack run {path_to_distribution_config} --image-type venv
```
It would raise an error:
```
RuntimeError: Failed to resolve 'tool_runtime' provider 'rag-runtime' of type 'inline::rag-runtime': required dependency 'files' is not available. Please add a 'files' provider to your configuration or check if the provider is properly configured.
```

This PR fixes the issue by adding missing files provider to NVIDIA
distribution.

## Test Plan
N/A

llama_stack/distributions/nvidia/build.yaml

@@ -23,6 +23,8 @@ distribution_spec:
     - provider_type: inline::basic
     tool_runtime:
     - provider_type: inline::rag-runtime
+    files:
+    - provider_type: inline::localfs
 image_type: venv
 additional_pip_packages:
 - aiosqlite