tar-util: allow to build without acl support (#39965)

This adds minimal definitions provided by libacl to make tar-util built
without libacl support.
Fixes the following compile error:
```
../src/shared/tar-util.c:62:9: error: unknown type name 'acl_t'
   62 |         acl_t acl_access, acl_default;
      |         ^~~~~
```

Follow-up for 4ded7f7a434c59534f65a0f9d391c55961eb110d.
Fixes #39928.
Replaces #39954.