Port away from deprecated QWeakPointer API

Instead of using weakptr.data(), now weakptr.toStrongRef().data() should be
used.

oxygen/kdecoration/oxygensettingsprovider.cpp:79:48: warning: ‘T* QWeakPointer<T>::data() const
 [with T = KDecoration2::DecoratedClient]’ is deprecated: Use toStrongRef() instead, and data() on
 the returned QSharedPointer [-Wdeprecated-declarations]
 auto client = decoration->client().data();