patch 8.1.1410: popup_move() is not implemented yet

Problem:    Popup_move() is not implemented yet.
Solution:   Implement it. (Yasuhiro Matsumoto, closes #4441)  Improve the
            positioning and resizing.
diff --git a/src/proto/popupwin.pro b/src/proto/popupwin.pro
index 64fca5f..f4d4760 100644
--- a/src/proto/popupwin.pro
+++ b/src/proto/popupwin.pro
@@ -8,4 +8,5 @@
 void popup_close_tabpage(tabpage_T *tp, int id);
 void close_all_popups(void);
 void ex_popupclear(exarg_T *eap);
+void f_popup_move(typval_T *argvars, typval_T *rettv);
 /* vim: set ft=c : */