QNX: Make use of _readdir64_r

Latest QNX versions support _readdir64_r. It will be used by
QFileSystemIterator now, when available to support large file
systems.

Change-Id: I40135f405ad72e1ba85c6a4cd282144dd8171bc6
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>