Enable serde's derive feature

We derive `serde` traits so we need the `derive` feature to be enabled
in all builds not just dev builds.

Presumably this bug has not shown up because anytime `serde` is enabled
some other crate in the stack enabled `derive`.

Cargo.toml

@@ -27,11 +27,10 @@ alloc = []
 [dependencies]
 arrayvec = { version = "0.7.2", default-features = false }
 core2 = { version = "0.3.2", default-features = false, optional = true }
-serde = { version = "1.0", default-features = false, optional = true }
+serde = { version = "1.0", default-features = false, features = ["alloc", "derive"], optional = true }
 
 
 [dev-dependencies]
-serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 
 [[example]]