Fix Issue 22453 - OpenBSD: Add a dummy value for AI_V4MAPPED

[ibara]

No description provided.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22453	minor	OpenBSD: Add a dummy value for AI_V4MAPPED

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "stable + druntime#3604"

[ibara]

Spurious dlang-community/dfmt test error?

[kinke]

It's the only target in this module not supporting it, and it's documented as 'required' in the header comment.