Add bridge handling for WDS STA interfaces

By default, add them to the configured bridge of the AP interface
(if present), but allow the user to specify a separate bridge.

src/ap/ap_config.h

@@ -148,6 +148,7 @@ struct hostapd_wmm_ac_params {
 struct hostapd_bss_config {
 	char iface[IFNAMSIZ + 1];
 	char bridge[IFNAMSIZ + 1];
+	char wds_bridge[IFNAMSIZ + 1];
 
 	enum hostapd_logger_level logger_syslog_level, logger_stdout_level;