Windows: add a couple of constants to WinSDK

Add some WinUser.h (User32) constants to WinSDK to make it easier to use
the WinSDK for more purposes (e.g. SwiftWin32).

Author: compnerd

stdlib/public/Windows/WinSDK.swift

@@ -41,3 +41,8 @@ public let FOF_NO_UI: FILEOP_FLAGS =
 public let INVALID_SOCKET: SOCKET = SOCKET(bitPattern: -1)
 public let FIONBIO: Int32 = 0x4667e
 
+// WinUser.h
+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)