sysutils/slurm-wlm: update to version 16.05.06

Changes in Slurm 16.05.6:
https://github.com/SchedMD/slurm/blob/master/NEWS

Port changes: Added a patch to fix a build issue. The patch was also
accepted upstream and should appear in the next Slurm release.

Reviewed by: mat
Approved by: swills (mentor)
Differential Revision: https://reviews.freebsd.org/D8411