debian/tests/control: systemd-repart requires libfdisk

If libfdisk-dev is not installed, systemd-repart will not be built, and post-install script in mkosi will fail.

4581s ‣  Copying in extra file trees…
4584s ‣  Running postinstall script /tmp/autopkgtest.HN91mk/build.PIo/systemd/mkosi.images/system/mkosi.postinst.chroot…
4584s /work/postinst: line 45: systemd-repart: command not found
4584s ‣ "/work/postinst final" returned non-zero exit code 127.
4587s FAILED: .