Add set_reuseaddr for Hermit shell

Was missing, but the code depends on it.

src/sys/shell/tcp.rs

@@ -21,7 +21,7 @@ pub(crate) fn listen(_: &net::TcpListener, _: u32) -> io::Result<()> {
     os_required!();
 }
 
-#[cfg(unix)]
+#[cfg(any(unix, target_os = "hermit"))]
 pub(crate) fn set_reuseaddr(_: &net::TcpListener, _: bool) -> io::Result<()> {
     os_required!();
 }