Revert "Remove all uses of G_CONST_RETURN"

This reverts commit 36741245cc.

The removal has not been discussed, except on Bugzilla:

https://bugzilla.gnome.org/show_bug.cgi?id=644611

glib/gunicode.h

@@ -236,7 +236,7 @@ typedef enum
  * in case the locale's charset will be changed later using setlocale()
  * or in some other way.
  */
-gboolean g_get_charset (const char **charset);
+gboolean g_get_charset (G_CONST_RETURN char **charset);
 
 /* These are all analogs of the <ctype.h> functions.
  */