netfilter: Kconfig: use 'default y' instead of 'm' for bool config option

This option, NF_CONNTRACK_SECMARK, is a bool, so it can never be 'm'.

Fixes: 33b8e776

 ("[NETFILTER]: Add CONFIG_NETFILTER_ADVANCED option")
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>