relibc: fix missing String import in socket.rs IPv6 formatting

Commit 79685cbe (AF_INET6 socket support) introduced
String::with_capacity() at socket.rs:80 but only imported
ToString from alloc::string, not String itself. This caused
E0433 'cannot find type String in this scope' during canonical
build (edition 2024 with -Z build-std).

Fix: add String to the existing alloc::string import.

src/platform/redox/socket.rs

@@ -1,4 +1,4 @@
-use alloc::{borrow::Cow, string::ToString, vec::Vec};
+use alloc::{borrow::Cow, string::{String, ToString}, vec::Vec};
 use core::{cmp, fmt::Write, mem, ptr, slice, str};
 use redox_path::RedoxStr;
 use redox_protocols::protocol::{FsCall, O_CLOEXEC, SocketCall};