uapi: nfnetlink_cthelper.h: fix userspace compilation error

Apparently, <linux/netfilter/nfnetlink_cthelper.h> and
<linux/netfilter/nfnetlink_acct.h> could not be included into the same
compilation unit because of a cut-and-paste typo in the former header.

Fixes: 12f7a505 ("netfilter: add user-space connection tracking helper infrastructure")
Cc: <stable@vger.kernel.org> # v3.6
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>