netfilter: conntrack: built-in support for UDPlite

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

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

(builtin)|| udplite|  ipv4  |  ipv6  |nf_conntrack
---------++--------+--------+--------+--------------
none     || 432538 | 828755 | 828676 | 6141434
UDPlite  ||   -    | 829649 | 829362 | 6498204

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