riched20: Constify some variables.
diff --git a/dlls/riched20/undo.c b/dlls/riched20/undo.c
index 2a9d915..d277e26 100644
--- a/dlls/riched20/undo.c
+++ b/dlls/riched20/undo.c
@@ -48,7 +48,7 @@
   } 
 }
 
-ME_UndoItem *ME_AddUndoItem(ME_TextEditor *editor, ME_DIType type, ME_DisplayItem *pdi) {
+ME_UndoItem *ME_AddUndoItem(ME_TextEditor *editor, ME_DIType type, const ME_DisplayItem *pdi) {
   if (editor->nUndoMode == umIgnore)
     return NULL;
   else if (editor->nUndoLimit == 0)