Satisfy the build/check-requires-timestamp task (see #785)

I think this line should not be required, as `ol.expression.Char` is only used internally in this file.

src/ol/expression/lexer.js

@@ -13,6 +13,7 @@
  * Copyright (C) 2011 Ariya Hidayat <ariya.hidayat@gmail.com>
  */
 
+goog.provide('ol.expression.Char'); // TODO: remove this - see #785
 goog.provide('ol.expression.Lexer');
 goog.provide('ol.expression.Token');
 goog.provide('ol.expression.TokenType');