kernel: netfilter.mk: fix kmod-ipt-nat6 installation on 5.4

xt_MASQUERADE.ko is picked up by both kmod-ipt-nat and kmod-ipt-nat6, causing
conflict
As kmod-ipt-nat6 already depends on kmod-ipt-nat, remove xt_MASQUERADE from it

Fixes: FS#2924
Fixes: 0fad8af8

 ("kernel: Include xt_MASQUERADE for kernel 5.2 and later")
Signed-off-by: DENG Qingfang <dengqf6@mail2.sysu.edu.cn>

include/netfilter.mk

@@ -196,7 +196,6 @@ $(eval $(if $(NF_KMOD),$(call nf_add,IPT_NAT,CONFIG_NETFILTER_XT_NAT, $(P_XT)xt_
 $(eval $(if $(NF_KMOD),$(call nf_add,IPT_NAT,CONFIG_IP_NF_NAT, $(P_V4)iptable_nat),))
 $(eval $(if $(NF_KMOD),$(call nf_add,IPT_NAT6,CONFIG_IP6_NF_NAT, $(P_V6)ip6table_nat),))
 $(eval $(if $(NF_KMOD),$(call nf_add,IPT_NAT6,CONFIG_IP6_NF_TARGET_MASQUERADE, $(P_V6)ip6t_MASQUERADE, lt 5.2),))
-$(eval $(if $(NF_KMOD),$(call nf_add,IPT_NAT6,CONFIG_IP6_NF_TARGET_MASQUERADE, $(P_XT)xt_MASQUERADE, ge 5.2),))
 $(eval $(if $(NF_KMOD),$(call nf_add,IPT_NAT6,CONFIG_IP6_NF_TARGET_NPT, $(P_V6)ip6t_NPT),))
 
 # userland only