backbone_babel: fix source addresses

roles/backbone_babel/templates/bird-backbone-babel.conf.j2

@@ -21,6 +21,7 @@ filter import_backbone_babel {
 {% endfor %}
 {% for network in bb_babel_networks|ansible.utils.ipv4 %}
 	if net.type = NET_IP4 && net ~ [{{ network }}+] then {
+		krt_prefsrc = {{ bb_babel_loopback_ipv4[0] }};
 		accept;
 	}
 {% endfor %}