evaluate: allow stateful statements with anonymous verdict maps

evaluate: allow stateful statements with anonymous verdict maps

Evaluation fails to accept stateful statements in verdict maps, relax
the following check for anonymous sets:

test.nft:4:29-35: Error: missing statement in map declaration
                ip saddr vmap { 127.0.0.1 counter : drop, * counter : accept }
                                          ^^^^^^^

The existing code generates correctly the counter in the anonymous
verdict map.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>