Windows: add some WinSock2 constants

This is the bare minimum of the WinSock2 constants that are needed for
the Foundation port to Windows which uses the socket APIs for
`-[NSTask run]` internal signalling.

Author: compnerd

stdlib/public/Windows/WinSDK.swift

@@ -37,3 +37,7 @@ public let INVALID_HANDLE_VALUE: HANDLE = HANDLE(bitPattern: -1)!
 public let FOF_NO_UI: FILEOP_FLAGS =
     FILEOP_FLAGS(FOF_SILENT | FOF_NOCONFIRMATION | FOF_NOERRORUI | FOF_NOCONFIRMMKDIR)
 
+// WinSock2.h
+public let INVALID_SOCKET: SOCKET = SOCKET(bitPattern: -1)
+public let FIONBIO: Int32 = 0x4667e
+