devel/ghub-devel: Mark MAKE_JOBS_UNSAFE

I have been receiving build error of this port from pkg-fallout. I
myself can't reproduce it. But I experienced similar problem with
devel/magit-devel and devel/forge-devel before and it was fixed by
disabling parallel build. So mark MAKE_JOBS_UNSAFE hoping it also
fixes build error of this port. At least it doesn't make things worse.