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/proto/popupwin.pro b/src/proto/popupwin.pro
index 63ff37a..b964fc0 100644
--- a/src/proto/popupwin.pro
+++ b/src/proto/popupwin.pro
@@ -47,4 +47,6 @@
 int popup_is_popup(win_T *wp);
 int popup_create_preview_window(void);
 void popup_close_preview(void);
+void popup_set_title(win_T *wp);
+void popup_update_preview_title(void);
 /* vim: set ft=c : */