krb5: Moved host from Curl_auth_create_gssapi_user_message() to be argument

krb5: Moved host from Curl_auth_create_gssapi_user_message() to be argument

For consistency with the spnego and oauth2 code moved the setting of
the host name outside of the Curl_auth_create_gssapi_user_messag()
function.

This will allow us to more easily override it in the future.