Add unix and bump index-state of ghc-extra-pkgs

Author: hamishmack

overlays/ghc-packages.nix

@@ -80,6 +80,7 @@ let
       ghc-boot     = "libraries/ghc-boot";
       ghc-heap     = "libraries/ghc-heap";
       libiserv     = "libraries/libiserv";
+      unix         = "libraries/unix";
       iserv        = "utils/iserv";
       remote-iserv = "utils/remote-iserv";
       iserv-proxy  = "utils/iserv-proxy";