chore: only use mysql when enabled

Author: dan-online

src/db/conn.rs

@@ -48,6 +48,7 @@ pub enum AnyConnection {
     /// ```
     /// mysql://user:password@localhost:3306/database
     /// ```
+    #[cfg(feature = "mysql")]
     Mysql(diesel::MysqlConnection),
     /// A connection to a SQLite database.
     ///