Fix 'already initialized constant' warning

What does this MR do?

When running specs locally, the following warning appears:

.../gitlab/lib/backup/files.rb:10: warning: already initialized constant Backup::Files::DEFAULT_EXCLUDE
.../gitlab/lib/backup/files.rb:10: warning: previous definition of DEFAULT_EXCLUDE was here

This MR fixes the warning, and is consistent with the solution described in https://docs.gitlab.com/ee/development/gotchas.html#solution-1

Does this MR meet the acceptance criteria?

Exploratory Testing

Rake task which exercises the modified classes still works successfully locally:

$ bundle exec rake gitlab:backup:artifacts:create
2020-09-24 23:51:20 -0700 -- Dumping artifacts ...
2020-09-24 23:51:20 -0700 -- done

Conformity

• Code review guidelines