Update MutexUnavailable.swift (swiftlang#74182)

Fixes the availability syntax for minimal platforms.
Resolves: rdar://129345303

Author: Azoy

stdlib/public/Synchronization/Mutex/MutexUnavailable.swift

@@ -31,7 +31,7 @@
 ///       }
 ///     }
 ///
-@available(unavailable, *, message: "Mutex is not available on this platform")
+@available(*, unavailable, message: "Mutex is not available on this platform")
 @frozen
 @_staticExclusiveOnly
 public struct Mutex<Value: ~Copyable>: ~Copyable {}