debian: add missing python-gtk2 dependency

qvm-mru-entry requires it.

Fixes QubesOS/qubes-issues#1567

debian/control

@@ -40,6 +40,7 @@ Depends:
     python-gi,
     python-xdg,
     python-dbus,
+    python-gtk2,
     qubes-utils (>= 3.1.3),
     sudo,
     systemd,