shiny/driver/internal/win32: Close window correctly to be able to re-open one after closing

This is an additional commit to
https://github.com/golang/exp/pull/6
https://go-review.googlesource.com/c/exp/+/171697

I forgot to add the modification of the syscall_windows.go that generates the zsyscall_windows.go. This is the missed change.

Change-Id: Ib22037f5c51d3185dbc88cda12df9c2da55a895b
GitHub-Last-Rev: 393f2689f0d37a4e2c5a7e4e009c7233339699a6
GitHub-Pull-Request: golang/exp#7
Reviewed-on: https://go-review.googlesource.com/c/exp/+/171998
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/shiny/driver/internal/win32/syscall_windows.go b/shiny/driver/internal/win32/syscall_windows.go
index 36e0dfe..d71536d 100644
--- a/shiny/driver/internal/win32/syscall_windows.go
+++ b/shiny/driver/internal/win32/syscall_windows.go
@@ -183,3 +183,4 @@
 //sys	_ScreenToClient(hwnd syscall.Handle, lpPoint *_POINT) (ok bool) = user32.ScreenToClient
 //sys   _ToUnicodeEx(wVirtKey uint32, wScanCode uint32, lpKeyState *byte, pwszBuff *uint16, cchBuff int32, wFlags uint32, dwhkl syscall.Handle) (ret int32) = user32.ToUnicodeEx
 //sys	_TranslateMessage(msg *_MSG) (done bool) = user32.TranslateMessage
+//sys	_UnregisterClass(lpClassName *uint16, hInstance syscall.Handle) (done bool) = user32.UnregisterClassW