posix: sys/stat.h: declare missing types in sys/stat.h

After adopting the `<sys/stat.h>` header from picolibc, there is
a possibility that the following types are not defined.

```cpp
typedef int dev_t;
typedef int ino_t;
typedef unsigned short nlink_t;
typedef unsigned short uid_t;
typedef unsigned short gid_t;
typedef unsigned long blksize_t;
typedef unsigned long blkcnt_t;
```

Of the above missing types, the oonly ones that are used today
in Zephyr are `blksize_t` and `blkcnt_t`.

Signed-off-by: Chris Friedt <cfriedt@meta.com>