[Trivial] [stable]: Fix C++ header regression wrt. StaticIfDeclaration

[kinke]

No description provided.

[dlang-bot]

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Testing this PR locally

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

dub fetch digger
dub run digger -- build "stable + dmd#8313"

[wilzbach]

Seems like stable is currently broken on the auto-tester, hopefully #8314 helps.

[wilzbach]

Stable should be green again now.