Make QCoreWlan plugin compile on 10.9

We take the path of least resistence and keep the old API code for 10.6 while
updating the code for 10.7 and newer. This means we have some code duplication.
It also means that we only compile the 10.6 code for QCoreWlanEngine when the
deploymen target is 10.6.

The 10.6 version file should be removed once we drop support for Snow Leopard.

Change-Id: If4702b155bcdb7522800bf99a4dd37d4efed803a
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>
Reviewed-by: Lorn Potter <lorn.potter@jollamobile.com>