ImplXMLSignatureListener doesn't have to implement
XSignatureVerifyResultListener, signatureVerified() is only called at
SignatureVerifierImpl::notifyResultListener(), on an object set by
SignatureVerifierImpl::addSignatureVerifyResultListener(). But
addSignatureVerifyResultListener() is only called from
XSecController::prepareSignatureToRead() where the argument is of type
XSecController.

Change-Id: I6741e6a4ce5cf800b09189cd8b7814cd72d602ab
Reviewed-on: https://gerrit.libreoffice.org/36504Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>