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/vim.h b/src/vim.h
index 4107f6b..631a57d 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -622,6 +622,7 @@
 #define POPF_DRAG	0x20	// popup can be moved by dragging
 #define POPF_RESIZE	0x40	// popup can be resized by dragging
 #define POPF_MAPPING	0x80	// mapping keys
+#define POPF_INFO	0x100	// used for info of popup menu
 
 #ifdef FEAT_TEXT_PROP
 # define WIN_IS_POPUP(wp) ((wp)->w_popup_flags != 0)