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

ocamlrep_marshal/Cargo.toml

@@ -17,7 +17,7 @@ doctest = false
 crate-type = ["lib", "staticlib"]
 
 [dependencies]
-bitflags = "2.4"
+bitflags = { version = "2.4", features = ["serde"] }
 ocamlrep = { path = "../ocamlrep" }
 
 [build-dependencies]