tools: ynl: add header guards for nlctrl

tools: ynl: add header guards for nlctrl

I "extracted" YNL C into a GitHub repo to make it easier
to use in other projects: https://github.com/linux-netdev/ynl-c

GitHub actions use Ubuntu by default, and the kernel headers
there are missing f329a0ebeaba ("genetlink: correct uAPI defines").
Add the direct include workaround for nlctrl.

Fixes: 768e044a5fd4 ("doc/netlink/specs: Add spec for nlctrl netlink family")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240315002108.523232-1-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>