Fixed a bug in tutorials caused by uninitialized ros::Time here.

visualization_tutorials/librviz_tutorial was crashing for me with an exception caused by ros::Time not being initialized.  The cause was a ROS_INFO_THROTTLE() call added with the stereo-support changes.  Apparently ROS_INFO_THROTTLE() does not use wall time, it must be using ros time.  Calling ros::Time::init() first seems harmless.  Merge if you agree. :)