WIndows: add an explicit cast

The constant value is imported as an unsigned value which means that we
cannot or the values together.  Add an explicit cast.

Author: compnerd

stdlib/public/Windows/WinSDK.swift

@@ -45,4 +45,4 @@ public let FIONBIO: Int32 = 0x4667e
 public let WS_OVERLAPPEDWINDOW: UINT =
     UINT(WS_OVERLAPPED | WS_CAPTION | WS_SYSMENU | WS_THICKFRAME | WS_MINIMIZEBOX | WS_MAXIMIZEBOX)
 public let WS_POPUPWINDOW: UINT =
-    UINT(WS_POPUP | WS_BORDER | WS_SYSMENU)
+    UINT(Int32(WS_POPUP) | WS_BORDER | WS_SYSMENU)