netfilter.mk: add version conditional for CONFIG_NF_CT_PROTO_GRE

Kernel commit 22fc4c4c9fd6 ("netfilter: conntrack: gre: switch module to
be built-in") moved the CT GRE code into the core nf_conntrack.ko module
and changed the CONFIG_NF_CT_PROTO_GRE option to boolean for kernel 5.1
and onwards.

CONFIG_NF_CT_PROTO_GRE at the moment has no prompt and can only be
selected by NF_CONNTRACK_PPTP

Fixes: FS#2990 (partial)
Ref: https://bugs.openwrt.org/index.php?do=details&task_id=2990

Signed-off-by: Tony Ambardar <[email protected]>
[note that the option now can not be enabled on its own]
Signed-off-by: Yousong Zhou <[email protected]>

include/netfilter.mk

@@ -219,7 +219,7 @@ $(eval $(call nf_add,NF_NATHELPER,CONFIG_NF_NAT_FTP, $(P_XT)nf_nat_ftp))
 $(eval $(call nf_add,NF_NATHELPER_EXTRA,CONFIG_NF_CONNTRACK_BROADCAST, $(P_XT)nf_conntrack_broadcast))
 $(eval $(call nf_add,NF_NATHELPER_EXTRA,CONFIG_NF_CONNTRACK_AMANDA, $(P_XT)nf_conntrack_amanda))
 $(eval $(call nf_add,NF_NATHELPER_EXTRA,CONFIG_NF_NAT_AMANDA, $(P_XT)nf_nat_amanda))
-$(eval $(call nf_add,NF_NATHELPER_EXTRA,CONFIG_NF_CT_PROTO_GRE, $(P_XT)nf_conntrack_proto_gre))
+$(eval $(call nf_add,NF_NATHELPER_EXTRA,CONFIG_NF_CT_PROTO_GRE, $(P_XT)nf_conntrack_proto_gre, lt 5.1))
 $(eval $(call nf_add,NF_NATHELPER_EXTRA,CONFIG_NF_NAT_PROTO_GRE, $(P_V4)nf_nat_proto_gre))
 $(eval $(call nf_add,NF_NATHELPER_EXTRA,CONFIG_NF_CONNTRACK_H323, $(P_XT)nf_conntrack_h323))
 $(eval $(call nf_add,NF_NATHELPER_EXTRA,CONFIG_NF_NAT_H323, $(P_V4)nf_nat_h323))