tools: net: package libynl for use in selftests

Support building the C YNL userspace library into one big static file.
We can then link selftests against it for easy to use C netlink
interface.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Link: https://patch.msgid.link/20240628003253.1694510-14-almasrymina@google.com


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