Add platforms attribute to umockdev

See #36707 to observe that the build fails. It depends on linux specific packages such as systemd.

pkgs/development/libraries/umockdev/default.nix

@@ -21,5 +21,6 @@ stdenv.mkDerivation rec {
     description = "Mock hardware devices for creating unit tests";
     license = licenses.lgpl2;
     maintainers = [ maintainers.ndowens ];
+    platforms = with platforms; linux;
   };
 }