debian: Move python-xdg to depends section in debian/control

Otherwise it `qubes-desktop-file-install` fails in minimal template

debian/control

@@ -25,6 +25,7 @@ Depends:
     psmisc,
     python2.7,
     python-gi,
+    python-xdg,
     qubes-utils (>= 3.0.1),
     sudo,
     systemd,
@@ -49,7 +50,6 @@ Recommends:
     network-manager-gnome,
     ntpdate,
     python-nautilus,
-    python-xdg,
     system-config-printer,
     tinyproxy,
     xsettingsd,