Fix SPDX-License-Identifier annotation

[robinkrahl]

libnitrokey uses the SPDX-License-Identifier: LGPL-3.0 annotation, but the LGPL-3.0 identifier has been deprecated acording to this list. The identifier should be LGPL-3.0-or-later instead.

[szszszsz]

To be set as LGPL-3.0-only.