sysutils/slurm-wlm: Enable slurmrestd

The port currently explicitly disables slurmrestd with
--disable-slurmrestd, with a comment stating that it does not build on
FreeBSD.

This restriction appears to be obsolete. With the current Slurm version,
slurmrestd builds successfully on FreeBSD without requiring any
additional portability patches.

This change removes --disable-slurmrestd and adds the resulting
slurmrestd executable and REST/OpenAPI plugins to the package plist.

PR: 298466
Sponsored by: UNIS Labs (vvd, commit patch)