debian: add Depends: qubesdb-vm

Make sure that qubesdb is configured (including service start) before
executing postinst of qubes-core-agent package, which will communicate
with qubesdb service.

Fixes QubesOS/qubes-issues#3951

debian/control

@@ -46,6 +46,7 @@ Depends:
     python-dbus,
     qubes-utils (>= 3.1.3),
     qubes-core-agent-qrexec,
+    qubesdb-vm,
     systemd,
     x11-xserver-utils,
     xdg-user-dirs,