Fix unused dependency in gazebo's strong_hash_derive

Summary:
```lang=text
Target `fbcode//buck2/gazebo/strong_hash_derive:_strong_hash_derive` has unused dependencies:
    fbsource//third-party/rust:proc-macro2: proc_macro2

Target `fbcode//buck2/gazebo/strong_hash_derive:strong_hash_derive-unittest` has unused dependencies:
    fbsource//third-party/rust:proc-macro2: proc_macro2

errors: 0; warnings: 2
```

Reviewed By: JakobDegen

Differential Revision: D72681967

fbshipit-source-id: 01e9e3b98e3d23ff367fa02b65ca318735305b57

Author: David Tolnay

gazebo/strong_hash_derive/Cargo.toml

@@ -10,6 +10,5 @@ version = "0.1.0"
 proc-macro = true
 
 [dependencies]
-proc-macro2 = { workspace = true }
 quote = { workspace = true }
 syn = { workspace = true }