Make QCoreWlan plugin compile on 10.9

Backport of 79ccb4fc from Qt 5:

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: I07284a2579ba127abdca660ad4b8573576756399
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>