Install gnome-packagekit in Dom0

This provides gpk-update-viewer that is used by qubes-watch-updates to
display a list of available updates for Dom0.

conf/comps-qubes.xml

@@ -61,6 +61,7 @@
       <packagereq type="mandatory">desktop-backgrounds-basic</packagereq>
       <packagereq type="mandatory">firstboot</packagereq>
       <packagereq type="mandatory">glx-utils</packagereq>
+      <packagereq type="mandatory">gnome-packagekit</packagereq>
       <packagereq type="mandatory">xorg-x11-drivers</packagereq>
       <packagereq type="mandatory">xorg-x11-server-Xorg</packagereq>
       <packagereq type="mandatory">xorg-x11-utils</packagereq>