netlink: specs: support unterminated-ok

ynl-gen-c.py supports check unterminated-ok, but the yaml schemas don't
have this key. Add this to the yaml files.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20240308081239.3281710-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>