config: Let server.cache.enable have a default too

Signed-off-by: Gergely Nagy <me@gergo.csillger.hu>

src/config.rs

@@ -36,6 +36,7 @@ pub struct ServerConfig {
 
 #[derive(Clone, Debug, Deserialize, Default)]
 pub struct CacheConfig {
+    #[serde(default)]
     pub enable: bool,
 }