patch 9.0.0092: plugins cannot change v:completed_item

Problem:    Plugins cannot change v:completed_item.
Solution:   Make v:completed_item writeable. (Shougo Matsushita,
            closes #10801)
diff --git a/src/evalvars.c b/src/evalvars.c
index 7e6bf7a..16dfd0d 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -111,7 +111,7 @@
     {VV_NAME("oldfiles",	 VAR_LIST), &t_list_string, 0},
     {VV_NAME("windowid",	 VAR_NUMBER), NULL, VV_RO},
     {VV_NAME("progpath",	 VAR_STRING), NULL, VV_RO},
-    {VV_NAME("completed_item",	 VAR_DICT), &t_dict_string, VV_RO},
+    {VV_NAME("completed_item",	 VAR_DICT), &t_dict_string, 0},
     {VV_NAME("option_new",	 VAR_STRING), NULL, VV_RO},
     {VV_NAME("option_old",	 VAR_STRING), NULL, VV_RO},
     {VV_NAME("option_oldlocal",	 VAR_STRING), NULL, VV_RO},