Add ostruct gem dependency for ruby >= 3.5

[teclator]

Problem

After an upgrade in Tumbleweed my system is now using ruby version 4 and the service started to fail because of missing ostruct dependency which is not part of Ruby anymore since version 3.5.

Solution

Added the ostruct gem dependency for ruby versions greater than 3.5

Testing

• Tested manually

[ancorgs]

The test are failing because the json validator is failing to fetch 'https://github.com/agama-project/agama/blob/master/rust/agama-lib/share/software.schema.json'

That's because software.schema.json is a separate file at the api-v2 branch, but not at the master one. Since we are planning to merge soon into master, we concluded we can live with that error.

Thus... LGTM