[Extractor] Exclude GPG encrypted data from being indexed

Summary:
application/pgp-encrypted may be encoded as base64 and thus inherits from
text/plain, but contains no extractable plaintext at all.

CCBUG: 386791

Reviewers: Baloo, Frameworks, ngraham, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D18851