Before this change, selecting only GPG keys for ODF encrpytion relied on
the fact, that all X.509 implementations (nss+mscrypt) implement listing
public keys (getAllCertificates) as a stub.
See previous change Ia6b8ecb9
Change-Id: Icb36a4cefd60b94407129ff73fe62f9250bfbee5
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/171639
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <thorsten.behrens@allotropia.de>
