services: plasma: Fix kded not being started.
`kded' is a daemon responsible for many features of the Plasma desktop, it
provides media keys handling, a system tray, 'Background Services' control
through 'kcmshell6 kcm_kded'...
* gnu/packages/kde-plasma.scm (plasma): Add `kded' to propagated inputs.
* gnu/services/desktop.scm (plasma-dbus-service): Add `kded' to the list of
services passed to `dbus-configuration'.
Change-Id: Ifffdaecdb28a6369727ab8d118a775d73ee342e9
Signed-off-by: Zheng Junjie <z572@z572.online>