TextView: Be const-correct when passing a pointer

The TextIter is passed by pointer for efficiency. We neither need to
modify it, nor should we leave it possible to accidentally do so. So,
it should be passed as a pointer-to-const.

gtk/gtktextview.c

@@ -5068,7 +5068,7 @@ move_cursor (GtkTextView       *text_view,
 }
 
 static gboolean
-iter_line_is_rtl (GtkTextIter *iter)
+iter_line_is_rtl (const GtkTextIter *iter)
 {
   GtkTextIter start, end;
   char *text;