patch 8.1.1880: cannot show extra info for completion in a popup window

Problem:    Cannot show extra info for completion in a popup window.
Solution:   Add the "popup" entry in 'completeopt'.
diff --git a/src/proto/ex_cmds.pro b/src/proto/ex_cmds.pro
index c31fee3..a0203e7 100644
--- a/src/proto/ex_cmds.pro
+++ b/src/proto/ex_cmds.pro
@@ -37,7 +37,7 @@
 char_u *get_old_sub(void);
 void set_old_sub(char_u *val);
 void free_old_sub(void);
-int prepare_tagpreview(int undo_sync);
+int prepare_tagpreview(int undo_sync, int use_previewpopup, int use_popup);
 void ex_help(exarg_T *eap);
 void ex_helpclose(exarg_T *eap);
 char_u *check_help_lang(char_u *arg);