Add xz-utils to our deb-recommends since it's necessary for some images

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)

hack/make/ubuntu

@@ -112,6 +112,7 @@ EOF
 		    --depends aufs-tools \
 		    --depends iptables \
 		    --deb-recommends ca-certificates \
+		    --deb-recommends xz-utils \
 		    --description "$PACKAGE_DESCRIPTION" \
 		    --maintainer "$PACKAGE_MAINTAINER" \
 		    --conflicts lxc-docker-virtual-package \