Fix missing feature

Author: stijnherfst

example/derive_expression/expression_lib/Cargo.toml

@@ -10,7 +10,7 @@ crate-type = ["cdylib"]
 
 [dependencies]
 num = "*"
-polars = { workspace = true, features = ["fmt", "dtype-date", "timezones"], default-features = false }
+polars = { workspace = true, features = ["fmt", "dtype-date", "timezones", "object"], default-features = false }
 polars-arrow = { workspace = true, default-features = false }
 pyo3 = { version = "0.24.2", features = ["abi3-py38"] }
 pyo3-polars = { version = "*", path = "../../../pyo3-polars", features = ["derive"] }