dirmngr: Only report hkps scheme when available

* dirmngr/ks-engine-hkp.c (ks_hkp_help): Make use of TLS macros.

Only report support for the hkps scheme when GnuPG / dirmngr
has been built with a TLS library.

This helps debuging and enable the user to detect whether support
for hkps is included by doing a
gpg-connect-agent --dirmngr 'keyserver --help' /bye.
Currently hkps will be listed as a supported scheme but trying to
add a keyserver using it will silently fail.

As a digression, https is never listed as a valid scheme.