MDEV-34348: Fix sql_list.h add_unique

Made base_list::add_unique templated, with a default
type of void (to adhere to existing uses), and then
the already templated class List can then be specific
about what types it will use in the provided comparator
function.

sql/sql_list.h

@@ -129,8 +129,6 @@ struct list_node :public Sql_alloc
   }
 };
 
-typedef bool List_eq(void *a, void *b);
-
 extern MYSQL_PLUGIN_IMPORT list_node end_of_list;
 
 class base_list :public Sql_alloc
@@ -301,11 +299,12 @@ class base_list :public Sql_alloc
   inline void **head_ref() { return first != &end_of_list ? &first->info : 0; }
   inline bool is_empty() { return first == &end_of_list ; }
   inline list_node *last_ref() { return &end_of_list; }
-  inline bool add_unique(void *info, List_eq *eq)
+  template <typename T= void>
+  inline bool add_unique(T *info, bool (*eq)(T *a, T *b))
   {
     list_node *node= first;
     for (;
-         node != &end_of_list && (!(*eq)(node->info, info));
+         node != &end_of_list && (!(*eq)((T *) node->info, info));
          node= node->next) ;
     if (node == &end_of_list)
       return push_back(info);
@@ -513,7 +512,7 @@ template <class T> class List :public base_list
   inline void prepend(List<T> *list) { base_list::prepend(list); }
   inline void disjoin(List<T> *list) { base_list::disjoin(list); }
   inline bool add_unique(T *a, bool (*eq)(T *a, T *b))
-  { return base_list::add_unique(a, (List_eq *)eq); }
+  { return base_list::add_unique<T>(a, eq); }
   inline bool copy(const List<T> *list, MEM_ROOT *root)
   { return base_list::copy(list, root); }
   void delete_elements(void)