muparserx: mark broken for darwin

pkgs/development/libraries/muparserx/default.nix

···
     homepage = "https://beltoforion.de/en/muparserx/";
     license = licenses.bsd2;
     maintainers = with maintainers; [ drewrisinger ];
+    # selftest fails
+    broken = stdenv.isDarwin;
   };
 }