netfilter.mk: add version conditional around nf_nat_redirect mod

Kernel commit 1ac89d20150e ("netfilter: nat: merge nf_nat_redirect into
nf_nat") made the redirect module part of the nat core and changed the
CONFIG_NF_NAT_REDIRECT option to a boolean, without prompt, affecting
kernel 4.18 onwards.  CONFIG_NF_NAT_REDIRECT now can only be selected by
CONFIG_NFT_REDIR or NETFILTER_XT_TARGET_REDIRECT

Fixes: FS#2476
Ref: https://bugs.openwrt.org/index.php?do=details&task_id=2476

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 has no prompt and can only be selected by other
kconfig options]
Signed-off-by: Yousong Zhou <[email protected]>

include/netfilter.mk

@@ -184,7 +184,7 @@ $(eval $(call nf_add,IPT_IPV6_EXTRA,CONFIG_IP6_NF_MATCH_RT, $(P_V6)ip6t_rt))
 
 # kernel only
 $(eval $(if $(NF_KMOD),$(call nf_add,NF_NAT,CONFIG_NF_NAT, $(P_XT)nf_nat),))
-$(eval $(if $(NF_KMOD),$(call nf_add,NF_NAT,CONFIG_NF_NAT_REDIRECT, $(P_XT)nf_nat_redirect),))
+$(eval $(if $(NF_KMOD),$(call nf_add,NF_NAT,CONFIG_NF_NAT_REDIRECT, $(P_XT)nf_nat_redirect, lt 4.18),))
 $(eval $(if $(NF_KMOD),$(call nf_add,NF_NAT,CONFIG_NF_NAT_IPV4, $(P_V4)nf_nat_ipv4, lt 5.1)))
 $(eval $(if $(NF_KMOD),$(call nf_add,NF_NAT6,CONFIG_NF_NAT_IPV6, $(P_V6)nf_nat_ipv6, lt 5.1)))