Add missing lock

Without change, slurmctld would abort running test38.1:

slurmctld: error: /home/jette/Desktop/SLURM/slurm.git/src/slurmctld/job_mgr.c:11177:
get_next_job_id(): Assertion (verify_lock(FED_LOCK, READ_LOCK)) failed.
Aborted (core dumped)