grel: Fix the internal type of GRealTuples.len to match the public API

`GTuples.len` is a `guint`.

This doesn’t seem to have caused any problems or warnings, but let’s get
it correct.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

glib/deprecated/grel.c

@@ -123,7 +123,7 @@ static size_t relation_count_internal (GRelation     *relation,
  **/
 struct _GRealTuples
 {
-  gint      len;
+  guint     len;
   gint      width;
   gpointer *data;
 };