patch 8.1.1416: popup_getposition() not implemented yet

Problem:    Popup_getposition() not implemented yet.
Solution:   Implement it. (Yasuhiro Matsumoto, closes #4449)
diff --git a/src/proto/popupwin.pro b/src/proto/popupwin.pro
index f4d4760..37b5273 100644
--- a/src/proto/popupwin.pro
+++ b/src/proto/popupwin.pro
@@ -3,6 +3,7 @@
 int popup_any_visible(void);
 void f_popup_close(typval_T *argvars, typval_T *rettv);
 void f_popup_hide(typval_T *argvars, typval_T *rettv);
+void f_popup_getposition(typval_T *argvars, typval_T *rettv);
 void f_popup_show(typval_T *argvars, typval_T *rettv);
 void popup_close(int id);
 void popup_close_tabpage(tabpage_T *tp, int id);