netfilter: conntrack: built-in support for SCTP

CONFIG_NF_CT_PROTO_SCTP is no more a tristate. When set to y, connection
tracking support for SCTP protocol is built-in into nf_conntrack.ko.

footprint test:
$ ls -l net/netfilter/nf_conntrack{_proto_sctp,}.ko \
        net/ipv4/netfilter/nf_conntrack_ipv4.ko \
        net/ipv6/netfilter/nf_conntrack_ipv6.ko

(builtin)||  sctp  |  ipv4  |  ipv6  | nf_conntrack
---------++--------+--------+--------+--------------
none     || 498243 | 828755 | 828676 | 6141434
SCTP     ||   -    | 829254 | 829175 | 6547872

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>