ynl: re-license uniformly under GPL-2.0 OR BSD-3-Clause

I was intending to make all the Netlink Spec code BSD-3-Clause
to ease the adoption but it appears that:
 - I fumbled the uAPI and used "GPL WITH uAPI note" there
 - it gives people pause as they expect GPL in the kernel
As suggested by Chuck re-license under dual. This gives us benefit
of full BSD freedom while fulfilling the broad "kernel is under GPL"
expectations.

Link: https://lore.kernel.org/all/20230304120108.05dd44c5@kernel.org/
Link: https://lore.kernel.org/r/20230306200457.3903854-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/lib/nlspec.py

@@ -1,4 +1,4 @@
-# SPDX-License-Identifier: BSD-3-Clause
+# SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
 
 import collections
 import importlib