debian: fix apt sources.list generation (missing debian version field)

Add Build-Depends: lsb-release, which is used for that.

debian/control

@@ -2,7 +2,7 @@ Source: qubes-core-agent
 Section: admin
 Priority: extra
 Maintainer: Davíð Steinn Geirsson <david@dsg.is>
-Build-Depends: qubes-utils (>= 2.0.17), libvchan-xen-dev, python, debhelper, quilt, libxen-dev, dh-systemd (>= 1.5)
+Build-Depends: qubes-utils (>= 2.0.17), libvchan-xen-dev, python, debhelper, quilt, libxen-dev, dh-systemd (>= 1.5), lsb-release
 Standards-Version: 3.9.5
 Homepage: http://www.qubes-os.org
 Vcs-Git: git://git.qubes-os.org/marmarek/core-agent-linux.git