fix formatting in defer.h

src/defer.h

@@ -4,6 +4,7 @@ struct Defer_Guard {
   ~Defer_Guard() { _f(); }
   F _f;
 };
+
 struct Defer_Guard_Helper {
   template <typename F>
   Defer_Guard<F> operator+(F &&f) { return Defer_Guard<F>(static_cast<F &&>(f)); }