Use proper build config for parsing proxy_arp

Use proper build config for parsing proxy_arp

In the definition of struct hostapd_bss_config, proxy_arp isn't affected
by the macro CONFIG_HS20. In addition, proxy_arp is not described in the
section of Hotspot 2.0 in the file hostapd.conf. The item proxy_arp
should be decided its action area by the macro CONFIG_PROXYARP which is
used to select whether the needed function gets included in the build.

Signed-off-by: Matt Woods <matt.woods@aliyun.com>