rule: don't list anonymous sets

rule: don't list anonymous sets

Don't list anonymous sets when listing all sets.

For example, using this ruleset:

==== 8< ====
table inet test {
	set set1 {
		type ipv4_addr
	}

	chain test {
		tcp dport { 80 } accept
	}
}
==== 8< ====

Before this patch:

% nft list sets
table inet test {
	set set0 {
		type inet_service
		flags constant
	}

	set set1 {
		type ipv4_addr
	}
}

After this patch:

% nft list sets
table inet test {
	set set1 {
		type ipv4_addr
	}
}

Fixes: 8f297010 ("rule: `list sets' only displays declaration, not definition")
Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>