patch 8.1.1673: cannot easily find the popup window at a certain position

Problem:    Cannot easily find the popup window at a certain position.
Solution:   Add popup_locate().
diff --git a/src/proto/popupwin.pro b/src/proto/popupwin.pro
index 7733368..d1267e0 100644
--- a/src/proto/popupwin.pro
+++ b/src/proto/popupwin.pro
@@ -29,6 +29,7 @@
 void f_popup_move(typval_T *argvars, typval_T *rettv);
 void f_popup_setoptions(typval_T *argvars, typval_T *rettv);
 void f_popup_getpos(typval_T *argvars, typval_T *rettv);
+void f_popup_locate(typval_T *argvars, typval_T *rettv);
 void f_popup_getoptions(typval_T *argvars, typval_T *rettv);
 int error_if_popup_window(void);
 void popup_reset_handled(void);