generate-ucl.lua: Add missing copyright header

Add a standard BSD license with a copyright claim for everyone who has
modified this file since it was added, excluding users who explicitly
declined attribution.

MFC after: 3 days
Reviewed by: ifreund_freebsdfoundation.org, imp, bapt, markj, emaste
Differential Revision: https://reviews.freebsd.org/D52881

(cherry picked from commit a6ac2c98c9cb22a4a2efc12cafce2d2ba82b3fdc)