ArgumentParser: repair the build after #504 (#506)

There are references to functions from the Windows SDK rather than the C
library for the console handling.  Explicitly import the necessary types
and functions from the WinSDK module so that Windows can build once more
with the changes to sequester the platform specific code.

Sources/ArgumentParser/Utilities/Platform.swift

@@ -41,7 +41,11 @@ extension Platform {
 // MARK: Exit codes
 
 #if os(Windows)
+import func WinSDK.GetStdHandle
+import func WinSDK.GetConsoleScreenBufferInfo
 import let WinSDK.ERROR_BAD_ARGUMENTS
+import let WinSDK.STD_OUTPUT_HANDLE
+import struct WinSDK.CONSOLE_SCREEN_BUFFER_INFO
 #endif
 
 extension Platform {