[dwrite] Try to fix delete-non-virtual-dtor warnings

Fixes #1548 hopefully
diff --git a/src/hb-directwrite.cc b/src/hb-directwrite.cc
index 0a73203..2739b88 100644
--- a/src/hb-directwrite.cc
+++ b/src/hb-directwrite.cc
@@ -133,8 +133,8 @@
 {
   IDWriteFactory *dwriteFactory;
   IDWriteFontFile *fontFile;
-  IDWriteFontFileStream *fontFileStream;
-  IDWriteFontFileLoader *fontFileLoader;
+  DWriteFontFileStream *fontFileStream;
+  DWriteFontFileLoader *fontFileLoader;
   IDWriteFontFace *fontFace;
   hb_blob_t *faceBlob;
 };