ynl: make the tooling check the license

The (only recently documented) expectation is that all specs
are under a certain license, but we don't actually enforce it.
What's worse we then go ahead and assume the license was right,
outputting the expected license into generated files.

Fixes: 37d9df22 ("ynl: re-license uniformly under GPL-2.0 OR BSD-3-Clause")
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>