fix build issue for density prior box op on windows test=develop

paddle/fluid/operators/detection/density_prior_box_op.cu

@@ -148,7 +148,7 @@ class DensityPriorBoxOpCUDAKernel : public framework::OpKernel<T> {
     // blockx is multiple of 32.
     int blockx = std::min(
         static_cast<int64_t>(((feature_width * num_priors + 31) >> 5) << 5),
-        512L);
+        static_cast<int64_t>(512L));
     int gridx = (feature_width * num_priors + blockx - 1) / blockx;
     dim3 threads(blockx, 1);
     dim3 grids(gridx, feature_height);