use WIN_StringToUTF8W macro instead of WIN_StringToUTF8, where needed:

i.e. where the string is known guaranteed to be WCHAR*, in:
- SDL_dinputjoystick.c (WIN_IsXInputDevice): VARIANT->var is BSTR (WCHAR*)
- SDL_rawinputjoystick.c (RAWINPUT_AddDevice): string is WCHAR*
- SDL_windows_gaming_input.c (IEventHandler_CRawGameControllerVtbl_InvokeAdded):
  string is WCHAR*

There should be more of these..

src/joystick/windows/SDL_windows_gaming_input.c

@@ -197,7 +197,7 @@ static HRESULT STDMETHODCALLTYPE IEventHandler_CRawGameControllerVtbl_InvokeAdde
                     if (SUCCEEDED(hr)) {
                         PCWSTR string = WindowsGetStringRawBufferFunc(hString, NULL);
                         if (string) {
-                            name = WIN_StringToUTF8(string);
+                            name = WIN_StringToUTF8W(string);
                         }
                         WindowsDeleteStringFunc(hString);
                     }