BigFix Patch module is included with any BigFix Platform subscription (Inventory, Compliance, Lifecycle or if you have the Patch module itself without any of the other 3 bundles) but it does require active subscription. ILMT itself will not entitle you to use Patch as it is free version without an active HCL subscription (I think it may officially be IBM tool although leveraging BigFix under the hood). Anyway, this document (page 3) should explain the different modules/offerings on BigFix side.
If you have the architecture (root server, relays, clients running on endpoints) as a part of ILMT/Inventory install then nothing else is required. The licensing will be enabled on the root server, the fixlets will become available in the BigFix console.