patch 9.1.0500: cannot switch buffer in a popup

Problem:  cannot switch buffer in a popup
          (Yggdroot)
Solution: add popup_setbuf() function

fixes: #15006
closes: #15026

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/proto/popupwin.pro b/src/proto/popupwin.pro
index 1ff9950..11679c6 100644
--- a/src/proto/popupwin.pro
+++ b/src/proto/popupwin.pro
@@ -34,6 +34,7 @@
 void popup_show(win_T *wp);
 void f_popup_show(typval_T *argvars, typval_T *rettv);
 void f_popup_settext(typval_T *argvars, typval_T *rettv);
+void f_popup_setbuf(typval_T *argvars, typval_T *rettv);
 int error_if_popup_window(int also_with_term);
 int popup_close(int id, int force);
 int popup_close_tabpage(tabpage_T *tp, int id, int force);