Import statement is not inserted when only a enum is used.
To reproduce compile bluetooth_hcd from https://github.com/kaitai-io/kaitai_struct_formats/pull/305 and see that https://github.com/kaitai-io/kaitai_struct_formats/pull/305/files#diff-d5dc77b92e2d98a1f9ab5b90cbdb3fe4R21 doesn't cause `BluetoothVendorsIds` be imported.
