[Issue 22598] importC: Add support for __extension__ keyword

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22598

--- Comment #1 from Iain Buclaw <ibuclaw at gdcproject.org> ---
I've worked around this by adding `#define __STRICT_ANSI__` to the sanitizer I
run with the preprocessor.

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