craterlake_tracking_only.yml: include far backward detectors (#767)

### Briefly, what does this PR introduce?
include far backward detectors in tracking only geometry.


### What kind of change does this PR introduce?
- [ ] Bug fix (issue #__)
- [ x] New feature (issue #__)
- [ ] Documentation update
- [ ] Other: __

### Please check if this PR fulfills the following:
- [ ] Tests for the changes have been added
- [ ] Documentation has been added / updated
- [ ] Changes have been communicated to collaborators

### Does this PR introduce breaking changes? What changes might users
need to make to their code?
N/A
### Does this PR change default behavior?
N/A

configurations/craterlake_tracking_only.yml

@@ -16,3 +16,5 @@ features:
     tof_endcap:
   far_forward:
     default:
+  far_backward:
+    default: