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

reverie/Cargo.toml

@@ -12,7 +12,7 @@ license = "BSD-2-Clause"
 addr2line = "0.21"
 anyhow = "1.0.75"
 async-trait = "0.1.71"
-bitflags = "2.4"
+bitflags = { version = "2.4", features = ["serde"] }
 byteorder = "1.3"
 lazy_static = "1.4"
 libc = "0.2.139"

safeptrace/Cargo.toml

@@ -9,7 +9,7 @@ repository = "https://github.com/facebookexperimental/reverie"
 license = "BSD-2-Clause"
 
 [dependencies]
-bitflags = "2.4"
+bitflags = { version = "2.4", features = ["serde"] }
 futures = { version = "0.3.28", features = ["async-await", "compat"] }
 lazy_static = "1.4"
 libc = "0.2.139"