MySQL 5.5 removed some dead code which includes last_used_con

Summary:
last_used_con was used for replication within libmysqlclient but this
was never enabled or used. MySQL developers killed it all in 5.5.0.

Use an #ifdef for now in case we're using some experimental stuff somewhere.

Test Plan:
Built against MySQL 5.5 and our external version

DiffCamp Revision: 177173
Reviewed By: hzhao
CC: hzhao, hphp-diffs@lists
Revert Plan:
Ok

src/runtime/ext/ext_mysql.cpp

@@ -709,7 +709,10 @@ static bool php_mysql_read_rows(MYSQL *mysql, CVarRef result) {
 }
 
 static Variant php_mysql_localize_result(MYSQL *mysql) {
+#if MYSQL_VERSION_ID < 50500
   mysql = mysql->last_used_con;
+#endif
+
   if (!mysql->fields) return true;
   if (mysql->status != MYSQL_STATUS_GET_RESULT) {
     // consistent with php_mysql_do_query_general