gnu: xfce: Use HTTPS for xfce.org home pages.

* gnu/packages/xfce.scm (gtk-xfce-engine, libxfce4util, xfconf, libxfce4ui)
(exo, garcon, tumbler, xfce4-panel, xfce4-battery-plugin)
(xfce4-clipman-plugin, xfce4-pulseaudio-plugin, xfce4-xkb-plugin)
(xfce4-appfinder, xfce4-session, xfce4-settings, thunar, thunar-volman)
(xfwm4, xfdesktop, xfce4-terminal, xfce, xfce4-power-manager, ristretto)
(xfce4-taskmanager, orage)[home-page]: Use HTTPS.