Fix build error of cpp unit test

Signed-off-by: wenjh <wenjh@sugon.com>

tests/cpp/operator/test_cast_mxfp8.cu

@@ -103,7 +103,7 @@ void compute_ref(const ProcessingMethod processing_method,
                     elt = static_cast<float>(static_cast<InputType>(elt));
 
                     cache_buffer[cache_idx] = elt;
-                    if (isinf(elt) || isnan(elt)) {
+                    if (std::isinf(elt) || std::isnan(elt)) {
                         continue;
                     }
                 }