debian: Do not compress the arch:all package with xz

We do not want to compress the arch:all package with xz, as that is not
supported on Lucid. To remedy the situation, explicitly pass -Zgzip to
dh_builddeb. For the sake of completeness, do this for the binary
packages too.

Reported-by: Imre Lazar <imre@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>