Add option to deactivate searching for gnome keyring

Summary:
The current cmake files search for gnome keyring unconditionally, which
on source-based distros would lead to a hard dependency on the package.
Hence, it would be a good idea to introduce an option to deactivate this
if necessary.

Reviewers: Falkon, drosca

Reviewed By: Falkon, drosca

Subscribers: heikobecker, drosca

Differential Revision: https://phabricator.kde.org/D8038