sasl_sspi: Fixed SPN not being converted to wchar under Unicode builds

Curl_sasl_create_digest_md5_message() would simply cast the SPN variable
to a TCHAR when calling InitializeSecurityContext(). This meant that,
under Unicode builds, it would not be valid wide character string.

Updated to use the recently introduced Curl_sasl_build_spn() function
which performs the correct conversion for us.