T7195: remove an accidentally committed object file and add a proper .gitignore

[dmbaturin]

Change Summary

A build artifact was accidentally committed to ipaddrcheck (https://github.com/vyos/ipaddrcheck/blob/current/src/ipaddrcheck_functions.o) — due to my double negligence: I didn't add a proper .gitignore to it from the start and didn't notice a .o file in git status before committing.

That was the cause of floating CI failures after the fix for T6744: the fix does work, but when a compiled object file is present, recursive make may assume that it doesn't need to be built, so an outdated code ends up in the binary, instead of the fixed code in ipaddrcheck_functions.c

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Other (please describe):

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Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly