Read host name just once

Add an empty dependency array to useEffect(), to go back to the
behaviour since before commit 4bbb291281.

Thanks to @subhoghoshX for spotting!

src/app.tsx

@@ -32,7 +32,7 @@ export const Application = () => {
         const hostname = cockpit.file('/etc/hostname');
         hostname.watch(content => setHostname(content?.trim() ?? ""));
         return hostname.close;
-    });
+    }, []);
 
     return (
         <Card>