QNX: fix call to realpath() on QFileSystemEngine

QNX: fix call to realpath() on QFileSystemEngine

QNX6, as opposed to later QNX versions, does not define the behavior of
realpath() when the second parameter is NULL. We leave it then for the
compiler to decide which implementation to call, by relying on the _POSIX_VERSION_
macro, which in QNX6 happens to evaluate to < 200801L.

Change-Id: I3a6d3859858f56b69f0d391384c2703083a01257
Reviewed-by: Qt Doc Bot <qt_docbot@qt-project.org>
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>