imgs/isp-common: fix access to DNS

This fixes commit: 57f0b251
It added Bind as ISP's DNS resolver but open in firewall was missing.

imgs/isp-common/etc/awall/optional/isp.yaml

@@ -12,6 +12,7 @@ policy:
 
 filter:
     - { in: local, out: _fw, service: ping, action: accept }
+    - { in: local, out: _fw, service: dns, action: accept }
 
 snat:
     - out: internet