Add mising header

[Hyxogen]

The file cheerpintrin.h uses size_t, but the necessary header for it is not included.

[yuri91]

maybe we should instead not use size_t but unsigned int in those two places.
This header is automatically included everywhere, so I don't like it to bring other stuff with it.

[Hyxogen]

This header is automatically included everywhere, so I don't like it to bring other stuff with it.

Any (working) code already includes one or more of these headers:

• cstddef
• cstdio
• cstring
• ctime
• cuchar
• cwchar

Or otherwise it would not compile because of those size_t. So I don't think it brings that much other stuff with it

[yuri91]

that's true. And it's not automatically included, just the search path for it is.

So I guess it's ok to add the include