gitingore: Add PERSISTENT_DIR to gitignore

* PERSISTENT_DIR was moved to TOPDIR in:
  http://git.openembedded.org/openembedded-core/commit/?
    id=5d43f752429707b74dbf46ecb81ac76bad7bb715

Change-Id: I403f9b6691354474a57d94d52578c0554f77924d