Update availability-macros.def

Author: Azoy

utils/availability-macros.def

@@ -33,6 +33,7 @@ SwiftStdlib 5.5:macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0
 SwiftStdlib 5.6:macOS 12.3, iOS 15.4, watchOS 8.5, tvOS 15.4
 SwiftStdlib 5.7:macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0
 SwiftStdlib 5.8:macOS 13.3, iOS 16.4, watchOS 9.4, tvOS 16.4
+SwiftStdlib 5.9:macOS 9999, iOS 9999, watchOS 9999, tvOS 9999
 
 # Local Variables:
 # mode: conf-unix