[][kernel][common][hnat][Fix mtkhnat build error]

[Description]
Fix build error.

If without this patch, the build of mtkhnat will fail if user select
both CONFIG_MEDIATEK_NETSYS_V3 and CONFIG_MEDIATEK_NETSYS_V2.

[Release-log]
N/A


Change-Id: I21f20dd4c04933fc0050bdb0ca253e233e81bab1
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/6598800
Build: srv_hbgsm110
1 file changed