Benchmark: Micro benchmark - Fix precision loss in kernel launch overhead benchmark

superbench/benchmarks/micro_benchmarks/kernel_launch_overhead/kernel_launch.cu

@@ -60,8 +60,8 @@ double test_cuda_kernel_launch_wall_time(int num_warmups, int num_steps) {
         EmptyKernel<<<1, 1>>>();
         cudaDeviceSynchronize();
         gettimeofday(&end_tv, NULL);
-        total_time += (((end_tv.tv_sec) * 1000 + (end_tv.tv_usec) / 1000) -
-                       ((begin_tv.tv_sec) * 1000 + (begin_tv.tv_usec) / 1000));
+        total_time += (end_tv.tv_sec - begin_tv.tv_sec) * 1000.0 +
+                      (end_tv.tv_usec - begin_tv.tv_usec) / 1000.0;
     }
 
     return total_time;