Add missing feature requirement

Author: DougGregor

test/Macros/default_main_actor_nonisolated.swift

@@ -6,6 +6,7 @@
 // Check for errors
 // RUN: %target-swift-frontend -swift-version 5 -typecheck -load-plugin-library %t/%target-library-name(MacroDefinition) %s -I %t -disable-availability-checking -swift-version 6 -default-isolation MainActor -enable-experimental-feature SendableProhibitsMainActorInference
 
+// REQUIRES: swift_feature_SendableProhibitsMainActorInference
 
 @attached(extension, conformances: Sendable)
 macro AddSendable() = #externalMacro(module: "MacroDefinition", type: "SendableMacro")