sddm: add patch for CVE-2020-28049

> Local privilege escalation due to race condition in creation of the Xauthority file.

Fixes: CVE-2020-28049
(cherry picked from commit faf436ea7915672835fce7a6b99576f491595d15)

pkgs/applications/display-managers/sddm/default.nix

···
-{ mkDerivation, lib, fetchFromGitHub
+{ mkDerivation, lib, fetchFromGitHub, fetchpatch
 , cmake, extra-cmake-modules, pkgconfig, libxcb, libpthreadstubs
 , libXdmcp, libXau, qtbase, qtdeclarative, qtquickcontrols2, qttools, pam, systemd
 }:
···
 
   patches = [
     ./sddm-ignore-config-mtime.patch
+    (fetchpatch {
+      name = "CVE-2020-28049.patch";
+      url = "https://github.com/sddm/sddm/commit/be202f533ab98a684c6a007e8d5b4357846bc222.patch";
+      sha256 = "119zqfldaqs4066wifn64n1xmhv1qd2acd9hcshw9hyl2j84pz8h";
+    })
   ];
 
   postPatch =