Codechange: Use functor for Kdtree's XYFunc. (#13074)

Kdtree uses a function pointer and incorrectly calls it a functor. The function pointer needs to be passed on instantiaton.

Instead, use an actual functor. This simplifies instantiation.

src/town_gui.cpp

@@ -50,7 +50,7 @@
 
 #include "safeguards.h"
 
-TownKdtree _town_local_authority_kdtree(&Kdtree_TownXYFunc);
+TownKdtree _town_local_authority_kdtree{};
 
 typedef GUIList<const Town*, const bool &> GUITownList;