WinSDK: extend the WinSock module further

This enables the use of `TransmitFile` more fully by adding the
Microsoft extensions to the WinSock module.

Author: compnerd

stdlib/public/Platform/winsdk.modulemap

@@ -14,6 +14,7 @@ module WinSDK [system] [extern_c] {
   module WinSock2 {
     header "WinSock2.h"
     header "WS2tcpip.h"
+    header "MSWSock.h"
     export *
 
     link "WS2_32.Lib"