Require dconf utility to (re)build /etc/dconf/db/local

Some applications complains if compiled version of dconf database is
missing ("dconf-WARNING **: unable to open file '/etc/dconf/db/local':
Failed to open file '/etc/dconf/db/local': open() failed: No such file
or directory; expect degraded performance").
There is only one entry in that database, but generate its binary
version anyway to avoid that warning message.

The dconf call is already included in package scripts, now only make
sure the utility is really installed.

QubesOS/qubes-issues#1951

debian/control

@@ -26,6 +26,7 @@ Vcs-Git: https://github.com/QubesOS/qubes-core-agent-linux
 Package: qubes-core-agent
 Architecture: any
 Depends:
+    dconf-cli,
     dmsetup,
     gawk,
     imagemagick,

rpm_spec/core-agent.spec.in

@@ -129,6 +129,7 @@ Requires:   python2-qubesdb
 Requires:   ImageMagick
 Requires:   librsvg2-tools
 Requires:   zenity
+Requires:   dconf
 Requires:   qubes-core-agent-qrexec
 Requires:   qubes-libvchan
 Requires:   qubes-db-vm