Remove 'const' that prevents Microsoft VC realize that

Item_trigger_field::set_required_privilege(const bool rw) is an
overloading of Settable_routine_parameter::set_required_privilege(bool rw).

sql/item.cc

@@ -5365,7 +5365,7 @@ bool Item_trigger_field::eq(const Item *item, bool binary_cmp) const
 }
 
 
-void Item_trigger_field::set_required_privilege(const bool rw)
+void Item_trigger_field::set_required_privilege(bool rw)
 {
   /*
     Require SELECT and UPDATE privilege if this field will be read and

sql/item.h

@@ -2187,7 +2187,7 @@ class Item_trigger_field : public Item_field,
   void cleanup();
 
 private:
-  void set_required_privilege(const bool rw);
+  void set_required_privilege(bool rw);
   bool set_value(THD *thd, sp_rcontext *ctx, Item *it);
 
 public: