Changes

contrib/jenkins.sh: specify --with-unitdir for nftables

Since recently (see the related commit), libnftables installs
a systemd unit file that loads a ruleset from a file on boot.

contrib/jenkins.sh currently fails with the following error:

  /usr/bin/install: cannot create regular file
                    '/lib/systemd/system/nftables.service':
                    Permission denied

Let's specify the unit directory like we do for other projects.

Change-Id: I9ab473dd6f734606518a980152fd4b00c2e23690
Related: nftables.git c4b17cf8305108f91d8e63880317a59e8e806c80