cleanup: make find_date_time_item() static

sql/item_cmpfunc.cc

@@ -42,7 +42,7 @@
   this is the type that will be used in warnings like
   "Incorrect <<TYPE>> value".
 */
-Item *find_date_time_item(Item **args, uint nargs, uint col)
+static Item *find_date_time_item(Item **args, uint nargs, uint col)
 {
   Item *date_arg= 0, **arg, **arg_end;
   for (arg= args, arg_end= args + nargs; arg != arg_end ; arg++)

sql/item_func.h

@@ -2507,7 +2507,6 @@ Item *get_system_var(THD *thd, enum_var_type var_type, LEX_STRING name,
 extern bool check_reserved_words(LEX_STRING *name);
 extern enum_field_types agg_field_type(Item **items, uint nitems,
                                        bool treat_bit_as_number);
-Item *find_date_time_item(Item **args, uint nargs, uint col);
 double my_double_round(double value, longlong dec, bool dec_unsigned,
                        bool truncate);
 bool eval_const_cond(COND *cond);