Version of Witness Server for mirroring SQL 2012
No witness server has to be SQL 2012, you can use SQL Express 2012 though

All server participating in database mirroring should be running same version of SQL server.

Taken from Microsoft SQL Server BOL


  Except during an upgrade from an earlier version of SQL Server, the
  server instances in a mirroring session must all be running the same
  version of SQL Server. For example, a SQL Server 2005 witness is
  supported when you are upgrading from a SQL Server 2005 mirroring
  configuration but cannot be added to an existing or new SQL Server
  2008 or later mirroring configuration.


Database Mirroring will be deprecated in future release of SQL Server if possible use AlwasyOn Availability Group