systemd: fix reproducibility of dbus interface xml

systemd's `--bus-introspect` was generating nondeterministic xml which
is saved into our build outputs

pkgs/os-specific/linux/systemd/default.nix

@@ -178,6 +178,13 @@ stdenv.mkDerivation {
     # need (AFAICT).
     # See https://github.com/systemd/systemd/pull/20479 for upstream discussion.
     ./0019-core-handle-lookup-paths-being-symlinks.patch
+
+    # fixes reproducability of dbus xml files
+    # Should no longer be necessary with v251.
+    (fetchpatch {
+      url = "https://github.com/systemd/systemd/pull/22174.patch";
+      sha256 = "sha256-RVhxUEUiISgRlIP/AhU+w1VHfDQw2W16cFl2TXXyxno=";
+    })
   ] ++ lib.optional stdenv.hostPlatform.isMusl (
     let
       oe-core = fetchzip {