xxkb: Fix 'lib.optional' use

pkgs/applications/misc/xxkb/default.nix

···
   buildInputs = [
     imake
     libX11 libXt libXext libXpm
-  ] ++ stdenv.lib.optional svgSupport [ librsvg glib gdk_pixbuf pkgconfig ];
+  ] ++ stdenv.lib.optionals svgSupport [ librsvg glib gdk_pixbuf pkgconfig ];
 
   outputs = [ "out" "man" ];