fix compiler warning

http://build.ros.org/view/Ldev/job/Ldev__libuvc_ros__ubuntu_xenial_amd64/3/warnings21Result/

libuvc_camera/src/camera_driver.cpp

@@ -246,7 +246,7 @@ void CameraDriver::AutoControlsCallback(
   void *data, size_t data_len) {
   boost::recursive_mutex::scoped_lock(mutex_);
 
-  printf("Controls callback. class: %d, event: %d, selector: %d, attr: %d, data_len: %u\n",
+  printf("Controls callback. class: %d, event: %d, selector: %d, attr: %d, data_len: %zu\n",
          status_class, event, selector, status_attribute, data_len);
 
   if (status_attribute == UVC_STATUS_ATTRIBUTE_VALUE_CHANGE) {