sasl: Fixed Kerberos V5 inclusion when CURL_DISABLE_CRYPTO_AUTH is used

Typically the USE_WINDOWS_SSPI definition would not be used when the
CURL_DISABLE_CRYPTO_AUTH define is, however, it is still a valid build
configuration and, as such, the SASL Kerberos V5 (GSSAPI) authentication
data structures and functions would incorrectly be used when they
shouldn't be.

Introduced a new USE_KRB5 definition that takes into account the use of
CURL_DISABLE_CRYPTO_AUTH like USE_SPNEGO and USE_NTLM do.