patch 8.1.1770: cannot get the window ID of the popup preview window

Problem:    Cannot get the window ID of the popup preview window.
Solution:   Add popup_getpreview().
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 3875b43..8253df8 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -715,6 +715,7 @@
     {"popup_filter_yesno", 2, 2, f_popup_filter_yesno},
     {"popup_getoptions", 1, 1, f_popup_getoptions},
     {"popup_getpos",	1, 1, f_popup_getpos},
+    {"popup_getpreview", 0, 0, f_popup_getpreview},
     {"popup_hide",	1, 1, f_popup_hide},
     {"popup_locate",	2, 2, f_popup_locate},
     {"popup_menu",	2, 2, f_popup_menu},