Avoid warning caused by async-trait 0.1.44

Summary:
With async-trait-0.1.42 this code worked great. With async-trait-0.1.44 it started generating code that upset clippy with:

```
   = note: requested on the command line with `-D clippy::let-unit-value`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_unit_value
help: omit the `let` binding
```

So start ignoring that lint warning.

Reviewed By: milend

Differential Revision: D26843687

fbshipit-source-id: 93ddad828e044ec112b4224ca068d34610fdf91e

starlark/bin/lsp.rs

@@ -67,6 +67,7 @@ impl Backend {
 }
 
 #[tower_lsp::async_trait]
+#[allow(clippy::let_unit_value)] // From the generated code
 impl LanguageServer for Backend {
     async fn initialize(&self, _: InitializeParams) -> Result<InitializeResult> {
         let mut r = InitializeResult::default();