Add explicit dependency between mapper and sdbus

Mapper has an implicit dependency on dbus.socket but
not on dbus.service. It appears an explicit dependency
is required to ensure the shutdown happens correctly.

Testing: Prior to this commit you would see this on a reboot:
Jul 20 15:01:34 witherspoon systemd[1]: Stopping D-Bus System MessageBus...
Jul 20 15:01:36 witherspoon systemd[1]: Stopped D-Bus System MessageBus.
Jul 20 15:01:37 witherspoon systemd[1]: xyz.openbmc_project.ObjectMapper.service: Main process exited,code=exited, status=1/FAILURE
Jul 20 15:01:37 witherspoon systemd[1]: xyz.openbmc_project.ObjectMapper.service: Failed with result'exit-code'.
Jul 20 15:01:41 witherspoon systemd[1]: Closed D-Bus System Message Bus Socket.

This shows the dbus.service stopping before the ObjectMapper

With this fix, this is now seen:
Jul 20 17:38:20 witherspoon systemd[1]: Stopping Phosphor DBus Service Discovery Manager...
Jul 20 17:38:21 witherspoon systemd[1]: Stopped Phosphor DBus Service Discovery Manager.
Jul 20 17:38:21 witherspoon systemd[1]: Stopping D-Bus System Message Bus...
Jul 20 17:38:21 witherspoon systemd[1]: Stopped D-Bus System Message Bus.
Jul 20 17:38:21 witherspoon systemd[1]: Closed D-Bus System Message Bus Socket.

This shows ObjectMapper stopping prior to dbus.service

Resolves openbmc/openbmc#3319

Change-Id: I466ec45f4582b3242688e8308d156961bb79cdc1
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>