readest: add dependencies for TTS

thanks to @eljamm, see:
https://github.com/NixOS/nixpkgs/pull/404705#discussion_r2076233872

pkgs/by-name/re/readest/package.nix

@@ -15,6 +15,7 @@
   nix-update-script,
   moreutils,
   jq,
+  gst_all_1,
 }:
 
 rustPlatform.buildRustPackage (finalAttrs: {
@@ -78,6 +79,11 @@ rustPlatform.buildRustPackage (finalAttrs: {
     gtk3
     librsvg
     openssl
+    # TTS
+    gst_all_1.gstreamer
+    gst_all_1.gst-plugins-base
+    gst_all_1.gst-plugins-good
+    gst_all_1.gst-plugins-bad
   ];
 
   preBuild = ''