Fix the unhandled symlink to directory conversion in python-ldap-dbg

Add a call to dpkg-maintscript-helper symlink_to_dir to handle properly
the replacement of /u/s/doc/python-ldap-dbg symlink by a directory, in
order to smoothen the stretch -> buster transition. (Closes: #905198)
- d/control: Add the Pre-Depends: ${misc:Pre-Depends} entry on
  python-ldap-dbg
- d/python-ldap-dbg.maintscript: Add the symlink_to_dir call.