curl_ntlm_core: Fixed use of long long for VC6 and VC7

Commit 07b66cbf unfortunately broke native NTLM message support in
compilers, such as VC6, VC7 and others, that don't support long long
type declarations. This commit fixes VC6 and VC7 as they support the
__int64 extension, however, we should consider an additional fix for
other compilers that don't support this.