nixos/knot: allow specifying the clear section

It was added in 3.3.4.  I'm not sure if it will be useful with NixOS
services, but I added it in the usual way anyway.

nixos/modules/services/networking/knot.nix

@@ -44,6 +44,7 @@ let
         ++ [ (sec_list_fa "id" nix_def "template") ]
         ++ [ (sec_list_fa "domain" nix_def "zone") ]
         ++ [ (sec_plain nix_def "include") ]
+        ++ [ (sec_plain nix_def "clear") ]
       );
 
     # A plain section contains directly attributes (we don't really check that ATM).