c/source-copyright: add ambiguous-paragraph-in-dep5-copyright

Flag DEP-5 paragraphs a License, a Copyright and no Files field.
While strictly speaking it is allowed (as a "stand-alone license
paragraph" with an extra "Copyright" field), it is almost certainly
not what people intended it to be.

Signed-off-by: Niels Thykier <niels@thykier.net>