netfilter: built-in NAT support for SCTP

CONFIG_NF_NAT_PROTO_SCTP is no more a tristate. When set to y, NAT
support for SCTP protocol is built-in into nf_nat.ko.

footprint test:

(nf_nat_proto_)           | sctp   || nf_nat
--------------------------+--------++--------
no builtin                | 428344 || 2241312
SCTP builtin              |   -    || 2597032

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