[Distributed] fix missing availability annotation

resolves rdar://161100405

test/ModuleInterface/distributed_no_redundant_conformance.swift

@@ -13,6 +13,7 @@ public actor Example {}
 
 // CHECK-NOT: extension TestResilient.Example : _Concurrency.Actor {}
 
+@available(SwiftStdlib 5.7, *)
 public distributed actor DistributedExample {
   public typealias ActorSystem = LocalTestingDistributedActorSystem
   distributed func example() {}