grn.h OS X: OS X does not have off64_t

OS X's `off_t` is already 64bit.

```c
/usr/include/sys/_types/_off_t.h
30:typedef __darwin_off_t		off_t;

/usr/include/sys/_types.h
110:typedef __int64_t	__darwin_off_t;		/* [???] Used for file sizes
*/
```