netfilter: nf_ct_sctp: move ip_ct_sctp away from UAPI

ip_ct_sctp is an internal structure, embedded by the union
nf_conntrack_proto to store sctp-specific information at conntrack
entries. It has no business with UAPI.

This patch moves it from UAPI to a saner place, together with similar
structs for other protocols.

Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>