nixos: fix evaluation

After commit 5e468b9, evaluation failed with:

error: undefined variable ‘dnsExtensionMechanism’ at
.../nixpkgs/nixos/modules/config/networking.nix:177:33

Changed files
+1 -1
nixos
modules
+1 -1
nixos/modules/config/networking.nix
···
'' + optionalString cfg.dnsSingleRequest ''
# only send one DNS request at a time
resolv_conf_options+=' single-request'
-
'' + optionalString dnsExtensionMechanism ''
+
'' + optionalString cfg.dnsExtensionMechanism ''
# enable extension mechanisms for DNS
resolv_conf_options+=' edns0'
'' + optionalString hasLocalResolver ''