sshd: exclude gssapi when building without cgo

MR !682 (merged) broke building without cgo enabled as it introduced a dependency on a Kerberos library. This can only be disabled at runtime and thus static builds of gitlab-sshd are no longer possible.

This change introduces an alternative implementation of the GSSAPI structure which just rejects attempts to use it. That alternative implementation gets automatically activated in case the user is building without cgo.