netfilter: conntrack: remove duplicated include from nf_conntrack_proto_udp.c

Remove duplicated include.

Fixes: c779e849 ("netfilter: conntrack: remove get_timeout() indirection")
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>