Missing header for 'KERN_PROC_PID'

Error generated trying to compile uwsgi on OpenBSD 5.6:
```core/logging.c:819: error: 'KERN_PROC_PID' undeclared (first use in this function)```

```#include <sys/sysctl.h>``` is needed for ‘KERN_PROC_PID’ on (at least) OpenBSD.