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/option.h b/src/option.h
index 29ecd8a..71a599e 100644
--- a/src/option.h
+++ b/src/option.h
@@ -503,6 +503,7 @@
 #endif
 EXTERN int	p_gd;		// 'gdefault'
 #ifdef FEAT_TEXT_PROP
+EXTERN char_u	*p_cpp;		// 'completepopup'
 EXTERN char_u	*p_pvp;		// 'previewpopup'
 #endif
 #ifdef FEAT_PRINTER