try to find the smb backend for cups as runtime dependency

Summary:
smbspool from samba is a backend for cups that is effectively required
to add samba printers. without it the UI will not show samba options,
which is expected but it is not immediately obvious what is missing

Test Plan: backend reported as found/notfound as expected

Reviewers: dantti, ngraham

Reviewed By: ngraham

Subscribers: kde-utils-devel

Differential Revision: https://phabricator.kde.org/D22950