[sftp] generate debug header at build time (this requires kf5.14)

Summary:
now use having boilerplate code sit around, pretty much the same stuff is
generated by ECM. KF 5.14 is ancient, so it should be good to require.

Reviewers: broulik

Reviewed By: broulik

Differential Revision: https://phabricator.kde.org/D15453