netfilter: fix missing dependencies for NETFILTER_XT_MATCH_CONNLABEL

It was possible to set NF_CONNTRACK=n and NF_CONNTRACK_LABELS=y via
NETFILTER_XT_MATCH_CONNLABEL=y.

warning: (NETFILTER_XT_MATCH_CONNLABEL) selects NF_CONNTRACK_LABELS which has
unmet direct dependencies (NET && INET && NETFILTER && NF_CONNTRACK)

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>