dbus: ensure directory exists before installing symlink

If you use a custom target skeleton with no /var/lib directory,
then the symlink installation in dbus.mk will fail.  This patch
fixes this issue by creating /var/lib before creating a symlink
there.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>