Fixed compile error caused by missing const declaration.

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@972 bbd45198-f89e-11dd-88c7-29a3b14d5316

components/rtgui/include/rtgui/font.h

@@ -73,7 +73,7 @@ struct rtgui_hz_file_font
     /* font file name */
     const char* font_fn;
 };
-extern struct rtgui_font_engine rtgui_hz_file_font_engine;
+extern const struct rtgui_font_engine rtgui_hz_file_font_engine;
 
 struct rtgui_font
 {