The disable_dgaf config fiels is only available in case Hostapd is
compiled with Hotspot 2.0 support, however Proxy-ARP does not depend on
Hotspot 2.0.
Only add the code related to this config field when Hotspot 2.0 is
enabled to fix compilation with the aformentioned preconditions.
Signed-off-by: David Bauer <mail@david-bauer.net>
