package/systemd: fix build on not-recent distros

package/systemd: fix build on not-recent distros

systemd is at it again, using ln --relative once more, which is not
available but on the most recent distros.

Patch it out with a bit of sed magic.

Fixes:
    http://autobuild.buildroot.org/results/48e/48ecf31eca9b22339a770abd3f9abdf1ae41cfd3/
    http://autobuild.buildroot.org/results/c62/c6297c61d8c2d3ab9be77b9d398598adacdb33b3/
    http://autobuild.buildroot.org/results/22f/22ff106a4626c84422d9a3e69aa9c8fe7c76938c/
    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>