Revert "tinc: remove unnecessary networking.interfaces"

This reverts commit cde886e.
This broke users using networking.interfaces since the interface
is not up when configuring addresses: #27070 (comment)

nixos/modules/services/networking/tinc.nix

@@ -148,6 +148,14 @@ in
         }
       ));
 
+    networking.interfaces = flip mapAttrs' cfg.networks (network: data: nameValuePair
+      ("tinc.${network}")
+      ({
+        virtual = true;
+        virtualType = "${data.interfaceType}";
+      })
+    );
+
     systemd.services = flip mapAttrs' cfg.networks (network: data: nameValuePair
       ("tinc.${network}")
       ({