Fix: Undefined variable 'this' should be 'self'

A trivial fix of the following error:

OfflineIMAP 6.5.5
  Licensed under the GNU GPL v2+ (v2 or any later version)
ERROR: Exceptions occurred during the run!
ERROR: While attempting to sync account 'sthu'
  global name 'this' is not defined

Traceback:
  File "/usr/lib64/python2.7/site-packages/offlineimap/accounts.py", line 241, in syncrunner
    self.sync()
  File "/usr/lib64/python2.7/site-packages/offlineimap/accounts.py", line 306, in sync
    remoterepos.sync_folder_structure(localrepos, statusrepos)
  File "/usr/lib64/python2.7/site-packages/offlineimap/repository/Base.py", line 187, in sync_folder_structure
    if src_folder.sync_this and not src_name_t in dst_folders:
  File "/usr/lib64/python2.7/site-packages/offlineimap/folder/Base.py", line 88, in sync_this
    return this.repository.should_sync_folder(self.ffilter_name)