Restore libslurm*-dev headers to /usr/include/slurm in bionic

[Nate Coraor]

Upstream patched the autoconf package name from `slurm` to `slurm-wlm` in order to fix a packaging bug, but the change also mistakenly relocated the headers from `/usr/include/slurm` to /usr/include/slurm-wlm`. Upstream also fixed this in a later change (which appears in cosmic) but that fix has not been included in bionic.

Original breaking change: https://salsa.debian.org/hpc-team/slurm-wlm/commit/08c387553a95e9a4a38047c82cf69a4f733cee0d

Upstream bug report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=887924

Upstream correction: https://salsa.debian.org/hpc-team/slurm-wlm/commit/d8815112340149742b3157a00927c2cb6fdf14b4

Both libslurm-dev and libslurmdb-dev are affected.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.