patch 8.1.0768: updating completions may cause the popup menu to flicker

Problem:    Updating completions may cause the popup menu to flicker.
Solution:   Avoid updating the text below the popup menu before drawing the
            popup menu.
diff --git a/src/proto/popupmnu.pro b/src/proto/popupmnu.pro
index f553100..e2ae92a 100644
--- a/src/proto/popupmnu.pro
+++ b/src/proto/popupmnu.pro
@@ -1,5 +1,7 @@
 /* popupmnu.c */
 void pum_display(pumitem_T *array, int size, int selected);
+void pum_call_update_screen(void);
+int pum_under_menu(int row, int col);
 void pum_redraw(void);
 void pum_undisplay(void);
 void pum_clear(void);