gnomeExtensions.gsconnect: fix build

dbus is now propagated from at-spi2-core, which made gsconnect try to
install dbus service files in the wrong location.

closes https://github.com/NixOS/nixpkgs/issues/79806

pkgs/desktops/gnome-3/extensions/gsconnect/default.nix

@@ -49,6 +49,7 @@ stdenv.mkDerivation rec {
     "-Dopenssl_path=${openssl}/bin/openssl"
     "-Dsshadd_path=${openssh}/bin/ssh-add"
     "-Dsshkeygen_path=${openssh}/bin/ssh-keygen"
+    "-Dsession_bus_services_dir=${placeholder "out"}/share/dbus-1/services"
     "-Dpost_install=true"
   ];