gnu: Use HTTPS for archive.xfce.org sources.
* gnu/packages/xfce.scm (gtk-xfce-engine, xfce4-xkb-plugin)
(xfce4-settings, thunar, ristretto, orage, xfburn, mousepad)
(xfce4-screenshooter, xfce4-screensaver, xfce4-cpugraph-plugin)
(xfce4-eyes-plugin, xfce4-equake-plugin, xfce4-datetime-plugin)
(xfce4-calculator-plugin, xfce4-cpufreq-plugin, xfce4-diskperf-plugin)
(xfce4-embed-plugin, xfce4-fsguard-plugin, xfce4-genmon-plugin)
(xfce4-kbdleds-plugin, xfce4-mailwatch-plugin, xfce4-mpc-plugin)
(xfce4-mount-plugin, xfce4-netload-plugin, xfce4-places-plugin)
(xfce4-smartbookmark-plugin, xfce4-statusnotifier-plugin)
(xfce4-stopwatch-plugin, xfce4-systemload-plugin)
(xfce4-time-out-plugin, xfce4-timer-plugin, xfce4-verve-plugin)
(xfce4-wavelan-plugin, xfce4-weather-plugin))[source]: Use HTTPS.
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