patch 8.1.1799: cannot avoid mapping for a popup window

Problem:    Cannot avoid mapping for a popup window.
Solution:   Add the "mapping" property, default TRUE.
diff --git a/src/proto/popupwin.pro b/src/proto/popupwin.pro
index b964fc0..f1d0d2b 100644
--- a/src/proto/popupwin.pro
+++ b/src/proto/popupwin.pro
@@ -38,6 +38,7 @@
 void popup_reset_handled(void);
 win_T *find_next_popup(int lowest);
 int popup_do_filter(int c);
+int popup_no_mapping(void);
 void popup_check_cursor_pos(void);
 void may_update_popup_mask(int type);
 void update_popups(void (*win_update)(win_T *wp));