readdir_r is deprecated since glibc version 2.24 and glibc manual
recomends usage of readdir instead of readdir_r.
Replacing readdir_r by readdir will also not be a problem for Nuttx,
because readdir_r is using readdir and filling parameters with the
return information.

Signed-off-by: Otavio Pontes <otavio.pontes@intel.com>