Add WinSDK import for sockets

Author: gmittert

TestFoundation/TestSocketPort.swift

@@ -6,6 +6,9 @@
 // See http://swift.org/LICENSE.txt for license information
 // See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 //
+#if os(Windows)
+import WinSDK
+#endif
 
 class TestPortDelegateWithBlock: NSObject, PortDelegate {
     let block: (PortMessage) -> Void