debian: install cvt tool

qubes.SetMonitorLayout service uses it.

Reported at QubesOS/qubes-video-companion#33 (comment)

Author: marmarek

debian/control

@@ -48,6 +48,7 @@ Depends:
     libpam-systemd,
     python3,
     python3-xcffib,
+    xcvt,
     xserver-xorg-video-dummyqbs (= ${binary:Version}),
     xserver-xorg-input-qubes (= ${binary:Version}),
     ${shlibs:Depends},