Foundation: map ECANCELED properly for Windows

This is generally not a very helpful constant for Windows, however, it
can be passed from Dispatch in some cases.  Lets just provide the
mapping for the Windows version.

Author: compnerd

Sources/Foundation/NSError.swift

@@ -1313,10 +1313,16 @@ extension POSIXError {
     public static var EBADMACHO: POSIXError.Code { return .EBADMACHO }
     #endif
 
-    #if !os(Windows)
     /// Operation canceled.
-    public static var ECANCELED: POSIXError.Code { return .ECANCELED }
+    public static var ECANCELED: POSIXError.Code {
+#if os(Windows)
+      return POSIXError.Code(rawValue: WinSDK.ECANCELED)!
+#else
+      return .ECANCELED
+#endif
+    }
 
+    #if !os(Windows)
     /// Identifier removed.
     public static var EIDRM: POSIXError.Code { return .EIDRM }