Window function create is hidden

The following warnings are generated when building the columnstore server. This is likely due to a incorrect function definition. It may cause hidden problems, or it may not. Either way, we strive to create warning free code.

In file included from /home/david/mariadb-columnstore-server/sql/item_create_window_function.cc:29:0:
/home/david/mariadb-columnstore-server/sql/item_create_window_function.h:72:16: warning: ‘virtual Item* Create_window_func_arg0::create(THD*, LEX_STRING, List<Item>*)’ was hidden [-Woverloaded-virtual]
virtual Item *create(THD *thd, LEX_STRING name, List<Item> *item_list)
^
/home/david/mariadb-columnstore-server/sql/item_create_window_function.cc:69:16: warning: by ‘virtual Item* Create_window_func_rank::create(THD*, LEX_STRING)’ [-Woverloaded-virtual]
virtual Item *create(THD *thd, LEX_STRING name)
^
In file included from /home/david/mariadb-columnstore-server/sql/item_create_window_function.cc:29:0:
/home/david/mariadb-columnstore-server/sql/item_create_window_function.h:72:16: warning: ‘virtual Item* Create_window_func_arg0::create(THD*, LEX_STRING, List<Item>*)’ was hidden [-Woverloaded-virtual]
virtual Item *create(THD *thd, LEX_STRING name, List<Item> *item_list)
^
/home/david/mariadb-columnstore-server/sql/item_create_window_function.cc:87:16: warning: by ‘virtual Item* Create_window_func_rownumber::create(THD*, LEX_STRING)’ [-Woverloaded-virtual]
virtual Item *create(THD *thd, LEX_STRING name)
^
In file included from /home/david/mariadb-columnstore-server/sql/item_create_window_function.cc:29:0:
/home/david/mariadb-columnstore-server/sql/item_create_window_function.h:116:16: warning: ‘virtual Item* Create_window_func_arg1::create(THD*, LEX_STRING, List<Item>*)’ was hidden [-Woverloaded-virtual]
virtual Item *create(THD *thd, LEX_STRING name, List<Item> *item_list)
^
/home/david/mariadb-columnstore-server/sql/item_create_window_function.cc:231:16: warning: by ‘virtual Item* Create_window_func_median::create(THD*, LEX_STRING, Item*)’ [-Woverloaded-virtual]
virtual Item *create(THD *thd, LEX_STRING name, Item *arg1)
^
In file included from /home/david/mariadb-columnstore-server/sql/item_create_window_function.cc:29:0:
/home/david/mariadb-columnstore-server/sql/item_create_window_function.h:116:16: warning: ‘virtual Item* Create_window_func_arg1::create(THD*, LEX_STRING, List<Item>*)’ was hidden [-Woverloaded-virtual]
virtual Item *create(THD *thd, LEX_STRING name, List<Item> *item_list)
^
/home/david/mariadb-columnstore-server/sql/item_create_window_function.cc:248:16: warning: by ‘virtual Item* Create_window_func_ntile::create(THD*, LEX_STRING, Item*)’ [-Woverloaded-virtual]
virtual Item *create(THD *thd, LEX_STRING name, Item *arg1)