Update to c++-style cast

Author: andrjohns

stan/math/fwd/functor/fvar_finite_diff.hpp

@@ -84,9 +84,10 @@ auto fvar_finite_diff(const F& func, const TArgs&... args) {
   FvarInnerT rtn_grad = 0;
   auto grad_deserializer = to_deserializer(grad);
   // Use a fold-expression to aggregate tangents for input arguments
-  (void)std::initializer_list<int>{(rtn_grad += internal::aggregate_tangent(
+  static_cast<void>(
+    std::initializer_list<int>{(rtn_grad += internal::aggregate_tangent(
                                         grad_deserializer.read(args), args),
-                                    0)...};
+                                    0)...});
 
   return FvarT(rtn_value, rtn_grad);
 }