net: geneve: fix array of flexible structures warnings

New compilers don't like flexible array of flexible structs:

  include/net/geneve.h:62:34: warning: array of flexible structures
Signed-off-by: NJakub Kicinski <kuba@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/geneve.h

@@ -59,7 +59,7 @@ struct genevehdr {
 	__be16 proto_type;
 	u8 vni[3];
 	u8 rsvd2;
-	struct geneve_opt options[];
+	u8 options[];
 };
 
 static inline bool netif_is_geneve(const struct net_device *dev)