vtls: Fixed compilation errors when SSL not used

Fixed the following warning and error from commit 3af90a6e when SSL
is not being used:

url.c:2004: warning C4013: 'Curl_ssl_cert_status_request' undefined;
            assuming extern returning int

error LNK2019: unresolved external symbol Curl_ssl_cert_status_request
               referenced in function Curl_setopt