agent: add required features for standalone build

Building the kata-agent-policy crate only succeeded when its parents
(agent and genpolicy) pulled in the required features. This commit adds
the required features to the crate itself, such that it can be built
standalone and IDEs don't show errors while browsing it.

Signed-off-by: Markus Rudy <mr@edgeless.systems>

src/agent/policy/Cargo.toml

@@ -7,7 +7,7 @@ license.workspace = true
 
 [dependencies]
 # Async runtime
-tokio.workspace = true
+tokio = { workspace = true, features = ["fs", "io-util"] }
 
 anyhow.workspace = true