Change: make the header guards all the same format

[mattmundell]

What

Use the same format for the header guards in .h files: #ifndef _GVM_<DIRNAME>_<FILENAME>

Why

1. It's neater
2. It's easier to understand what to use for new files.
3. It may prevent clashes with headers from other libraries.
4. It may prevent clashes when there are files with the same name in multiple dirs.

Testing

Compiles fine.

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Snapshot Warnings

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Scanned Files

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[bjoernricks]

🚀