patch 8.1.1882: cannot specify properties of the info popup window

Problem:    Cannot specify properties of the info popup window.
Solution:   Add the 'completepopup' option.  Default to PmenuSel highlight.
diff --git a/src/proto/popupwin.pro b/src/proto/popupwin.pro
index f2cbf07..0fad5ae 100644
--- a/src/proto/popupwin.pro
+++ b/src/proto/popupwin.pro
@@ -11,6 +11,7 @@
 int popup_extra_width(win_T *wp);
 void popup_adjust_position(win_T *wp);
 int parse_previewpopup(win_T *wp);
+int parse_completepopup(win_T *wp);
 void popup_set_wantpos_cursor(win_T *wp, int width);
 void popup_set_wantpos_rowcol(win_T *wp, int row, int col);
 void f_popup_clear(typval_T *argvars, typval_T *rettv);