Hook up XcmHandler to the system module (#381)

cumulus/rococo-parachains/runtime/src/lib.rs

@@ -233,8 +233,8 @@ impl cumulus_pallet_parachain_system::Config for Runtime {
 	type Event = Event;
 	type OnValidationData = ();
 	type SelfParaId = parachain_info::Module<Runtime>;
-	type DownwardMessageHandlers = ();
-	type HrmpMessageHandlers = ();
+	type DownwardMessageHandlers = XcmHandler;
+	type HrmpMessageHandlers = XcmHandler;
 }
 
 impl parachain_info::Config for Runtime {}