[][openwrt][mt7988][hnat][Add header guard in hnat.h]

[Description]
Add header guard in hnat.h

[Release-log]
N/A

Change-Id: Iad128cfb98116c0ac415402ef58df3312fa1f641
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/9058590
1 file changed