dnsapi: add DnsGetCacheDataTable stub

[Sebastian Lackner]

On 20.05.2016 06:27, Austin English wrote:
> +/******************************************************************************
> + * DnsGetCacheDataTable                    [DNSAPI.@]
> + *
> + */
> +DNS_GET_CACHE_DATA_TABLE WINAPI DnsGetCacheDataTable( DNSCACHEENTRY *cache )

I'm pretty sure it should not return a function pointer here. Also, you might want
to move the struct to header file so that it can be used in tests later.