netlink: specs: ethtool: add empty enum stringset

C does not allow defining structures and enums with the same name.
Since enum ethtool_stringset exists in the uAPI we need to include
at least a stub of it in the spec. This will trigger name collision
avoidance in the code gen.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>