Modify the code for creating a new directory

[zxytql]

In file "src/ob_camera_node.cpp" using a function boost::filesystem::create_directory to create a new directory:

    if (!boost::filesystem::exists(current_path + "/point_cloud")) {
      boost::filesystem::create_directory(current_path + "/point_cloud");
    }

and there are two other places. However, when compiling on some platforms, an undefined reference error will occur. #1 This problem can be solved by using the mkdir function in C language.