include: Add missing interface declaration.

[Nikolay Sivov]

>   interface IWbemContext;
>   interface IWbemServices;
> +interface IWbemLocator;
>   interface IWbemStatusCodeText;
>   interface IWbemCallResult;
>   interface IWbemObjectSink;
Why do you need this? IWbemLocator is fully defined later in this file.