sddm: ignore config file modification time

Properly fixes #10211, wherein the config file was not being loaded
because it was considered too old. However, the modification time was
being compared against an uninitialized value; the semantics of the
uninitialized timestamp probably changed between Qt 5.4 and 5.5.

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

@@ -14,6 +14,8 @@ stdenv.mkDerivation rec {
     sha256 = "09amr61srvl52nvxlqqgs9fzn33pc2gjv5hc83gxx43x6q2j19gg";
   };
 
+  patches = [ ./sddm-ignore-config-mtime.patch ];
+
   nativeBuildInputs = [ cmake pkgconfig qttools ];
 
   buildInputs = [ libxcb libpthreadstubs libXdmcp libXau qtbase qtdeclarative pam systemd ];