Fix build errors (MinGW-w64)

Fix build errors (MinGW-w64)

- rrd.h, fixed build error:
  rrd.h:66:17: error: conflicting types for 'ssize_t'
  typedef size_t ssize_t;
  Remark: MinGW-w64 has ssize_t and off_t
- rrd_restore.c, fixed build error:
  rrd_restore.c:29:17: error: conflicting types for 'ssize_t'
  typedef size_t ssize_t;
- rrd_utils.c, fixed build error:
  rrd_utils.c:226:10: error: too many arguments to function 'mkdir'
     if ((mkdir(pathname, mode) != 0) && (errno != EEXIST)) {