patch 8.1.1892: missing index entry and option menu for 'completepopup'

Problem:    Missing index entry and option menu for 'completepopup'.
Solution:   Add the entries.  Adjust #ifdefs to avoid dead code.
diff --git a/src/option.h b/src/option.h
index 0461a64..0ccf8f1 100644
--- a/src/option.h
+++ b/src/option.h
@@ -503,7 +503,9 @@
 #endif
 EXTERN int	p_gd;		// 'gdefault'
 #ifdef FEAT_TEXT_PROP
+# ifdef FEAT_QUICKFIX
 EXTERN char_u	*p_cpp;		// 'completepopup'
+# endif
 EXTERN char_u	*p_pvp;		// 'previewpopup'
 #endif
 #ifdef FEAT_PRINTER