meta-quanta: gbs: install obmc-mapper.target

Some services need obmc-mapper.target to start in correct
order, such as phosphor-virtual-sensors.

ref:
https://gerrit.openbmc-project.xyz/43952
https://gerrit.openbmc-project.xyz/43535



Signed-off-by: George Hung <george.hung@quantatw.com>
Change-Id: Ia463c9f189d715336626ce9e46cad0a370e9b8dd