#include <sys/types.h> should be before #include <sys/sysctl.h>

Here: https://github.com/indygreg/python-zstandard/blob/main/c-ext/backend_c.c#L16

The manpage sysctl(3) says:
```
SYNOPSIS
     #include <sys/types.h>
     #include <sys/sysctl.h>

     int
     sysctl(const int *name, u_int namelen, void *oldp, size_t *oldlenp,
         const void *newp, size_t newlen);
```