xfce4-session: add consolekit to RDEPENDS

This error was hidden for years by my test image containing consolekit.

One result of missing consolekit are greyed 'Restart' and 'Shut Down' in XFCE's
logout-dialog and the log message:

** (xfce4-session:4125): WARNING **: 'CanStop' method failed : The name org.freedesktop.ConsoleKit was not provided by any .service files

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>