x509-cert: use alloc instead of std

Author: dishmaker

x509-cert/src/ext/pkix/name/dirstr.rs

@@ -1,6 +1,6 @@
-use std::{borrow::Cow, string::ToString};
-
+use alloc::borrow::Cow;
 use alloc::string::String;
+use alloc::string::ToString;
 use der::{
     Choice, FixedTag, Header, Reader, ValueOrd,
     asn1::{Any, BmpString, PrintableString, TeletexString},