bitflags: enable feature 'serde'

Summary:
building the "renderdag" package (fbcode/eden/scm/lib/renderdag/Cargo.toml) fails with
```lang=php,counterexample
error[E0277]: the trait bound `InternalBitFlags: Serialize` is not satisfied
   --> /data/users/shaynefletcher/fbsource/fbcode/eden/scm/lib/renderdag/src/render.rs:213:46
```
this diff resolves the issue by enabling the `serde` feature on the bitflags crate

Reviewed By: capickett

Differential Revision: D54067851

fbshipit-source-id: 2f6ababbae0c4667e4896f03a2fb39c973de535c

vendor/ocaml/interop/shim/third-party/rust/Cargo.toml

@@ -13,7 +13,7 @@ path = "top/main.rs"
 
 [dependencies]
 anyhow = "1.0.75"
-bitflags = "2.4"
+bitflags = { version = "2.4", features = ["serde"] }
 bstr = { version = "1.4.0", features = ["serde", "std", "unicode"] }
 bumpalo = { version = "3.14.0", features = ["allocator_api", "collections"] }
 clap-3 = { package = "clap", version = "3.2.25", features = ["derive", "env", "regex", "unicode", "wrap_help"] }