package/sysrepo: fix install path for systemd units

package/sysrepo: fix install path for systemd units

The relative paths currently used to install the systemd units causes an
incorrect installation, where units are installed in (notice the double
usr in the paths):

    target/usr/usr/lib/systemd/system/sysrepod.service
    staging/usr/usr/lib/systemd/system/sysrepod.service

Fix that by using an absolute path.

Signed-off-by: Jérémy Rosen <jeremy.rosen@smile.fr>
[yann.morin.1998@free.fr:
  - split into its own commit
  - expand commit log
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>