kldxref: Add a copyright and license statement to ef.h

This uses the statement from other files in kldxref when the tool was
first imported in commit 9c6f92408c29a8980e7aec26a2590087c3b3d8ed.

Reviewed by: kevans
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42963