netfilter: select LIBCRC32C together with SCTP conntrack

nf_conntrack needs to compute crc32c when dealing with SCTP packets.
Moreover, NF_NAT_PROTO_SCTP (currently selecting LIBCRC32C) can be enabled
only if conntrack support for SCTP is enabled. Therefore, move enabling of
kernel support for crc32c so that it is selected when NF_CT_PROTO_SCTP=y.

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Reviewed-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>