ynl-gen-c.py: render netlink policies static for split ops

When policies are rendered for split ops, they are consumed in the same
file. No need to expose them for user outside, make them static.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20230803111340.1074067-5-jiri@resnulli.us


Signed-off-by: Jakub Kicinski <kuba@kernel.org>