PT-2375 - fixed pt-table-sync for tables with generated columns

Previously, pt-table-sync generated DML statements that included
the generated columns of a table, which is however rejected by
the database and not necessary for the required sync statements.

t/pt-table-sync/samples/pt-2375.sql

@@ -0,0 +1,12 @@
+DROP DATABASE IF EXISTS test;
+CREATE DATABASE test;
+USE test;
+
+CREATE TABLE `test_table` (
+  `id` INT AUTO_INCREMENT PRIMARY KEY,
+  `value` INT NOT NULL,
+  `derived_value` INT AS (2*`value`)
+) ENGINE=InnoDB;
+
+INSERT INTO `test_table` (`value`) VALUES (24);
+INSERT INTO `test_table` (`value`) VALUES (42);