dirent.h: Add d_ino member to struct dirent

This commit adds the `d_ino` member (`ino_t` type) to struct dirent
to make it compatible with the POSIX definition of the structure.
According to https://pubs.opengroup.org/onlinepubs/9799919799/, the
structure `dirent` shall include the following members:

```
ino_t  d_ino       File serial number.
char   d_name[]    Filename string of entry.
```

https://www.man7.org/linux/man-pages/man3/readdir.3.html also
states that:

"
Only the fields d_name and (as an XSI extension) d_ino are
specified in POSIX.1.  Other than Linux, the d_type field is
available mainly only on BSD systems.  The remaining fields are
available on many, but not all systems.
"

Although `d_ino` isn't being used by NuttX directly, the structure
`dirent` may be used by third-party applications and it's important
to have all the required members defined to avoid compatibility
issues.

Author: tmedicci

include/dirent.h

@@ -112,6 +112,7 @@
 
 struct dirent
 {
+  ino_t d_ino;                    /* file number */
   uint8_t  d_type;                /* Type of file */
   char     d_name[NAME_MAX + 1];  /* File name */
 };