style: format code with Rustfmt

This commit fixes the style issues introduced in 1da5b69 according to the output
from Rustfmt.

Details: #653

src/core/config.rs

@@ -31,8 +31,7 @@ pub struct BackupSettings {
     pub backup_state: String,
 }
 
-#[derive(Debug, Serialize, Deserialize, Clone)]
-#[derive(Default)]
+#[derive(Debug, Serialize, Deserialize, Clone, Default)]
 pub struct DeviceSettings {
     /// Unique serial identifier
     pub device_id: String,
@@ -52,7 +51,6 @@ impl Default for GeneralSettings {
     }
 }
 
-
 #[dynamic]
 static CONFIG_FILE: PathBuf = CONFIG_DIR.join("config.toml");