fix: correctly configure 'serde' features of dependencies (#33).

Due to surprising an to me still quite unclear behaviour `cargo`
will turn on the `serde` features in the `hex` and `smartstring`
dependencies when building it locally from a repository clone,
but won't do it when building the crates-io version of the crate
after adding it as dependency.

This fixes the issue by correctly configuring it the `serde`
feature.

Cargo.toml

@@ -37,8 +37,8 @@ http-reqwest = ["git-repository/blocking-http-transport-reqwest-rust-tls"]
 [dependencies]
 git-repository = { version = "0.29.0", default-features = false, features = ["max-performance-safe", "blocking-network-client"] }
 serde = { version = "1", features = ["std", "derive"] }
-hex = "0.4.3"
-smartstring = "1.0.1"
+hex = { version = "0.4.3", features = ["serde"] }
+smartstring = { version = "1.0.1", features = ["serde"] }
 serde_json = "1"
 bstr = "1.0.1"
 thiserror = "1.0.32"