reverie-syscalls: Fix broken OSS CI

Summary:
My recent diff: D49742979 which upgraded `bitflags` to 2.4 seems to have broken the Open Source CI for a few crates under `hermetic_infra`.

That is because among other changes, `bitflags` 2.x requires the `serde` Cargo feature to `#[derive(Serialize, Deserialize)]` on generated flag types.

See [the release notes](https://github.com/bitflags/bitflags/releases/tag/2.0.0?fbclid=IwAR0BHMz6T_jmIkXaJmkdvc9LdDTW15G5SSU_A-cZvLxof0aGS0BA7Hqnme0)

This change should fix it.

Reviewed By: mitrandir77

Differential Revision: D52801745

fbshipit-source-id: 826c70e5a111ab1a1159cceab6fb40831f355287

detcore/Cargo.toml

@@ -30,7 +30,7 @@ path = "tests/time/mod.rs"
 [dependencies]
 anyhow = "1.0.75"
 async-trait = "0.1.71"
-bitflags = "2.4"
+bitflags = { version = "2.4", features = ["serde"] }
 chrono = { version = "0.4", features = ["clock", "serde", "std"], default-features = false }
 clap = { version = "3.2.25", features = ["derive", "env", "regex", "unicode", "wrap_help"] }
 detcore-model = { version = "0.0.0", path = "../detcore-model" }