netfilter: nf_nat: define nat_pptp_info as needed

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>

include/net/netfilter/nf_nat.h

@@ -56,7 +56,9 @@ struct nf_nat_multi_range_compat {
 /* per conntrack: nat application helper private data */
 union nf_conntrack_nat_help {
 	/* insert nat helper private data here */
+#if defined(CONFIG_NF_NAT_PPTP) || defined(CONFIG_NF_NAT_PPTP_MODULE)
 	struct nf_nat_pptp nat_pptp_info;
+#endif
 };
 
 struct nf_conn;