Code review comment for lp:~widelands-dev/widelands/constructionsite_options

I get these compiler warnings a couple of times.

/Users/toni/Launchpad/widelands-repo/working_tree/src/logic/map_objects/tribes/building_settings.h:42:8: warning: 'Widelands::BuildingSettings' has virtual functions but non-virtual destructor [-Wnon-virtual-dtor]
struct BuildingSettings {
       ^
/Users/toni/Launchpad/widelands-repo/working_tree/src/logic/map_objects/tribes/building_settings.h:57:8: warning: 'Widelands::ProductionsiteSettings' has virtual functions but non-virtual destructor [-Wnon-virtual-dtor]
struct ProductionsiteSettings : public BuildingSettings {
       ^
/Users/toni/Launchpad/widelands-repo/working_tree/src/logic/map_objects/tribes/building_settings.h:74:8: warning: 'Widelands::MilitarysiteSettings' has virtual functions but non-virtual destructor [-Wnon-virtual-dtor]
struct MilitarysiteSettings : public BuildingSettings {
       ^
/Users/toni/Launchpad/widelands-repo/working_tree/src/logic/map_objects/tribes/building_settings.h:86:8: warning: 'Widelands::TrainingsiteSettings' has virtual functions but non-virtual destructor [-Wnon-virtual-dtor]
struct TrainingsiteSettings : public ProductionsiteSettings {
       ^
/Users/toni/Launchpad/widelands-repo/working_tree/src/logic/map_objects/tribes/building_settings.h:97:8: warning: 'Widelands::WarehouseSettings' has virtual functions but non-virtual destructor [-Wnon-virtual-dtor]
struct WarehouseSettings : public BuildingSettings {
       ^
5 warnings generated.