Add bin/gitlab-sshd as an explicit Makefile target

Since !682 (merged), Kerberos headers and libraries are needed to build gitlab-sshd. If they are not available, make build successfully compiles bin/gitlab-shell but fails to build bin/gitlab-sshd. However, running make build again would do nothing and appear to be succeed because bin/gitlab-shell existed. This led to Omnibus GitLab quietly dropping the gitlab-sshd binary, as seen in omnibus-gitlab!6446 (comment 1265879416).

To ensure make build properly fails if bin/gitlab-sshd cannot be built, we make the binary an explicit build target.