Added "const" to new data for handler::update_row()

This was done to make it clear that a update_row() should not change the
row.
This was not done for handler::write_row() as this function still needs
to update auto_increment values in the row. This should at some point
be moved to handler::ha_write_row() after which write_row can also have
const arguments.

storage/perfschema/table_setup_objects.h

@@ -74,7 +74,7 @@ protected:
 
   virtual int update_row_values(TABLE *table,
                                 const unsigned char *old_buf,
-                                unsigned char *new_buf,
+                                const unsigned char *new_buf,
                                 Field **fields);
 
   virtual int delete_row_values(TABLE *table,