[NETFILTER] PPTP helper: Add missing Kconfig dependency

PPTP should not be selectable without conntrack enabled
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/netfilter/Kconfig

@@ -139,6 +139,7 @@ config IP_NF_AMANDA
 
 config IP_NF_PPTP
 	tristate  'PPTP protocol support'
+	depends on IP_NF_CONNTRACK
 	help
 	  This module adds support for PPTP (Point to Point Tunnelling
 	  Protocol, RFC2637) connection tracking and NAT.