patch 8.1.1438: some commands cause trouble in a popup window

Problem:    Some commands cause trouble in a popup window.
Solution:   Add NOT_IN_POPUP_WINDOW.
diff --git a/src/proto/popupwin.pro b/src/proto/popupwin.pro
index eb9e241..107a427 100644
--- a/src/proto/popupwin.pro
+++ b/src/proto/popupwin.pro
@@ -13,4 +13,5 @@
 void f_popup_move(typval_T *argvars, typval_T *rettv);
 void f_popup_getpos(typval_T *argvars, typval_T *rettv);
 void f_popup_getoptions(typval_T *argvars, typval_T *rettv);
+int not_in_popup_window(void);
 /* vim: set ft=c : */