[Issue 24429] [ImportC] improve error message for size_t

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24429

--- Comment #2 from Lance Bachmeier <lance at lancebachmeier.com> ---
Same for NULL. The error message is incorrect: Error: undefined identifier
`NULL`, did you mean `null`? But if you change it to null, you get Error:
undefined identifier `null`.

Maybe stddef.h should always be included with C code. There's not much in
there, but what's there is used frequently.

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