tools: ynl: protect from old OvS headers

Since commit 7c59c9c8 ("tools: ynl: generate code for ovs families")
we need relatively recent OvS headers to get YNL to compile.
Add the direct include workaround to fix compilation on less
up-to-date OSes like CentOS 9.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240226225806.1301152-1-kuba@kernel.org


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