build: add missing require

Author: Planeshifter

lib/node_modules/@stdlib/_tools/pkgs/toposort/lib/async.js

@@ -22,6 +22,7 @@
 
 var pkgNames = require( '@stdlib/_tools/pkgs/names' );
 var isFunction = require( '@stdlib/assert/is-function' );
+var format = require( '@stdlib/string/format' );
 var copy = require( '@stdlib/utils/copy' );
 var validate = require( './validate.js' );
 var sort = require( './sort.js' );