Suppress unexpected_cfgs warning.

The cfg is valid as it's intended to be passed manually to rustc.

Fixes #21.

Author: comex

Cargo.toml

@@ -21,3 +21,6 @@ rust-version = "1.46.0"
 [features]
 std = []
 default = ["std"]
+
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(manual_codegen_check)'] }