calm down -Wdelete-non-virtual-dtor warnings

remove -Wno-non-virtual-dtor from makefile (what does it do anyway? -Wdelete-non-virtual-dtor is part of -Wall)

libs/entitylib.h

@@ -308,7 +308,7 @@ void render( RenderStateFlags state ) const {
 ///
 /// - Notifies observers when value changes - value changes to "" on destruction.
 /// - Provides undo support through the global undo system.
-class KeyValue : public EntityKeyValue
+class KeyValue final : public EntityKeyValue
 {
 typedef UnsortedSet<KeyObserver> KeyObservers;