pipewire: also handle pipewire-media-session in systemd class

For the package pipewire-media-session there is a service file
specified with SYSTEMD_SERVICE:${PN}-media-session but the package
was not added to SYSTEMD_PACKAGES and not handled in do_package by
systemd bbclass.

Signed-off-by: Peter Bergin <peter@berginkonsult.se>