patch 9.0.1515: reverse() does not work for a String

Problem:    reverse() does not work for a String.
Solution:   Implement reverse() for a String. (Yegappan Lakshmanan,
            closes #12179)
diff --git a/src/list.c b/src/list.c
index ca43526..7042965 100644
--- a/src/list.c
+++ b/src/list.c
@@ -2999,6 +2999,8 @@
 
     if (argvars[0].v_type == VAR_BLOB)
 	blob_reverse(argvars[0].vval.v_blob, rettv);
+    else if (argvars[0].v_type == VAR_STRING)
+	string_reverse(argvars[0].vval.v_string, rettv);
     else if (argvars[0].v_type != VAR_LIST)
 	semsg(_(e_argument_of_str_must_be_list_or_blob), "reverse()");
     else