qt: servicesdiscoverymodel: add missing override
Commit 2212ef87 added getSortFunction as a virtual pure function, but the class was not updated to override it.
Fixes the following warning:
../modules/gui/qt/network/servicesdiscoverymodel.cpp:141:50: warning: 'getSortFunction' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
141 | LocalListCacheLoader<SDItemPtr>::ItemCompare getSortFunction() const
| ^
../modules/gui/qt/util/locallistbasemodel.hpp:39:59: note: overridden virtual function is here
39 | virtual typename LocalListCacheLoader<T>::ItemCompare getSortFunction() const = 0;
| ^