library: Install xz-utils, a dep of gdebi-core

virtualbox-amd64-dev builds have been failing because gdebi is unable to extract and read the control file inside the custom freedombox* packages.

gdebi-core suggests 'xz-utils | xz-lzma' but neither package is being installed automatically.

Signed-off-by: Joseph Nuthalapati njoseph@riseup.net