scylla-cql: fix unit tests with 'serde' feature enabled

When running scylla-cql tests separately, serde 'derive' feature is not
set explicitly. This results in failing builds when the feature is enabled.
Builds were not failing earlier due to 'scylla-cql' being used as a dependency
of a crate that sets 'serde/derive' feature.

scylla-cql/Cargo.toml

@@ -24,7 +24,7 @@ num-bigint = "0.3"
 chrono = { version = "0.4", default-features = false }
 lz4_flex = { version = "0.9.2" }
 async-trait = "0.1.57"
-serde = { version = "1.0", optional = true }
+serde = { version = "1.0", features = ["derive"], optional = true }
 
 [dev-dependencies]
 criterion = "0.3"