should not lock tables when backing up mysql 5.5 database performance_schema

In the same fashion that --skip-lock-tables is used when backing up information_schema, mysql 5.5 needs for performance_schema

Warning: mysqldump: Got error: 1142: SELECT,LOCK TABL command denied to user 'debian-sys-maint'@'localhost' for table 'cond_instances' when using LOCK TABLES Warning: Failed to dump mysql databases performance_schema

patch attached.

(from redmine: created on 2012-01-10, closed on 2012-01-24)