Update the nl80211.h file in iw with the version from backports.
The files were out of sync already before the mac80211 update. If iw set
the NL80211_ATTR_WIPHY_ANTENNA_GAIN attribute the kernel assumed it set
the NL80211_ATTR_PUNCT_BITMAP attribute because the id was the same.
Link: https://github.com/openwrt/openwrt/pull/15827
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>