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>

Documentation/netlink/specs/ethtool.yaml

@@ -12,6 +12,10 @@ definitions:
     enum-name:
     type: enum
     entries: [ vxlan, geneve, vxlan-gpe ]
+  -
+    name: stringset
+    type: enum
+    entries: []
 
 attribute-sets:
   -