revert "discord: remove libunity dependency" (NixOS#445254)

Author: Artturin

pkgs/applications/networking/instant-messengers/discord/linux.nix

@@ -54,6 +54,7 @@
   pipewire,
   python3,
   runCommand,
+  libunity,
   speechd-minimal,
   wayland,
   branch,
@@ -152,6 +153,7 @@ stdenv.mkDerivation (finalAttrs: {
       libnotify
       libX11
       libXcomposite
+      libunity
       libuuid
       libXcursor
       libXdamage