RTEMS: Define ssize_t for <dirent.h>

This fixes the following build issue:

newlib/libc/include/dirent.h:84:1: error: unknown type name 'ssize_t'; did you mean '_ssize_t'?
   84 | ssize_t posix_getdents(int, void *, size_t, int);
      | ^~~~~~~
      | _ssize_t

newlib/libc/sys/rtems/include/sys/dirent.h

@@ -49,6 +49,11 @@ typedef	__off_t		off_t;
 #define	_OFF_T_DECLARED
 #endif
 
+#ifndef _SSIZE_T_DECLARED
+typedef _ssize_t ssize_t;
+#define	_SSIZE_T_DECLARED
+#endif
+
 typedef struct _dirdesc {
 	int	dd_fd;
 	long	dd_loc;