[xml] EISDIR is unknown to win32

["Vakoc, Mark" <Mark_Vakoc jdedwards com>]

xmlCheckFilename() in xmlIO.c was changed recently to use EISDIR, but that
is undefined on Win32.  Simple solution is to use the value 21 instead of
EISDIR or define EISDIR somewhere win32 specific.


For reference, from MSDN

The following is a listing of all the errno values defined in ERRNO.H along
with brief descriptions of their meanings. Only the values marked with an
asterisk (*) are considered valid under MS-DOS. Those marked with a plus
sign (+) are considered valid under Windows NT. 

For more information on the values not marked with an asterisk or a plus
sign, see a UNIX or XENIX system manual. 

Value       Define       Description
-----       ------       -----------

EPERM         1          Not owner
ENOENT        2        *+No such file or directory
ESRCH         3          No such process
EINTR         4          Interrupted system call
EIO           5          I/O error
ENXIO         6          No such device or address
E2BIG         7        *+Argument list too long
ENOEXEC       8        *+Exec format error
EBADF         9        *+Bad file number
ECHILD       10         +No spawned processes
EAGAIN       11         +No more processes; not enough memory;
                           maximum nesting level reached
ENOMEM       12        *+Not enough memory
EACCES       13        *+Permission denied
EFAULT       14          Bad address
ENOTBLK      15          Block device required
EBUSY        16          Mount device busy
EEXIST       17        *+File exists
EXDEV        18        *+Cross-device link
ENODEV       19          No such device
ENOTDIR      20          Not a directory
EISDIR       21          Is a directory
EINVAL       22        *+Invalid argument
ENFILE       23          File table overflow
EMFILE       24        *+Too many open files
ENOTTY       25          Not a teletype
ETXTBSY      26          Text file busy
EFBIG        27          File too large
ENOSPC       28        *+No space left on device
ESPIPE       29          Illegal seek
EROFS        30          Read-only file system
EMLINK       31          Too many links
EPIPE        32          Broken pipe
EDOM         33        *+Math argument
ERANGE       34        *+Result too large
EUCLEAN      35          File system needs cleaning
EDEADLK      36         +Resource deadlock would occur
EDEADLOCK    36         *Resource deadlock would occur
ENAMETOOLONG 38
ENOLCK       39
ENOSYS       40
ENOTEMPTY    41
EILSEQ       42

* Used under both MS-DOS
+ Used under Windows NT