patch 8.1.1789: cannot see file name of preview popup window

Problem:    Cannot see file name of preview popup window.
Solution:   Add the file name as the title.
diff --git a/src/fileio.c b/src/fileio.c
index 1bf4d87..bc0b15b 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -6101,6 +6101,9 @@
     }
     status_redraw_all();
     redraw_tabline = TRUE;
+#ifdef FEAT_TEXT_PROP
+    popup_update_preview_title();
+#endif
 }
 
 #if (defined(FEAT_DND) && defined(FEAT_GUI_GTK)) \