Mark Item_func_vector_distance as final class

Author: HuXin0817

sql/item_strfunc.h

@@ -1326,7 +1326,7 @@ class Item_func_from_vector final : public Item_str_ascii_func {
   String *val_str_ascii(String *str) override;
 };
 
-class Item_func_vector_distance : public Item_real_func {
+class Item_func_vector_distance final : public Item_real_func {
  public:
   Item_func_vector_distance(const POS &pos, Item *ilist1, Item *ilist2,
                             Item *ilist3)