privacyidea: build on linux only

I'm not even aware of darwin support, but since it appears to break[1],
I'd say it's skip it.

ZHF #230712

[1] https://hydra.nixos.org/build/219600077

pkgs/applications/misc/privacyidea/default.nix

@@ -242,5 +242,6 @@ python3'.pkgs.buildPythonPackage rec {
     license = licenses.agpl3Plus;
     homepage = "http://www.privacyidea.org";
     maintainers = with maintainers; [ globin ma27 ];
+    platforms = platforms.linux;
   };
 }