Missing include stddef.h - Xcode 16 B 1

[DntPullALockett]

Testing Xcode 16 Beta 1:

u8-check is missing stddef.h and it is also complaining about the use of NULL being a undeclared identifier.

I know that this may be a bug in Xcode 16 B1 but if others can also validate that would be great.

[jerrodputman]

I'm seeing it too. I submitted a pull request with the fix (one line), though I'm not sure why Xcode 16 is complaining about that include missing but 15 is not.

[DntPullALockett]

I think it has to do with Swift 6 checks