Add empty dependency array to useEffect hook

This limits the file watch listener to only attach once and not on every
render which is unnecessary.

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>