netfilter.mk: add version conditional around nf_nat_redirect mod

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 <itugrok@yahoo.com>
[note that the option has no prompt and can only be selected by other
kconfig options]
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>