[NETFILTER] PPTP helper: Fix compilation of conntrack helper without NAT

This patch fixes compilation of the PPTP conntrack helper when NAT is
configured off.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>