complex: Use 'import com.sun.star.beans.UnknownPropertyException;'

Change-Id: Ib42baf6f18fd407a879313c042efd74c8f724aa4
Reviewed-on: https://gerrit.libreoffice.org/11864
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>