Update submodule sitl_gazebo to latest Mon Dec 3 00:37:44 UTC 2018

    - sitl_gazebo in PX4/Firmware (99c225dd): https://github.com/PX4/sitl_gazebo/commit/7c97eeae139d86b3747ea877d8067fb3180dc4f1
    - sitl_gazebo current upstream: https://github.com/PX4/sitl_gazebo/commit/d08eb5b22d3edfb7a740c883b06f895a173b5519
    - Changes: https://github.com/PX4/sitl_gazebo/compare/7c97eeae139d86b3747ea877d8067fb3180dc4f1...d08eb5b22d3edfb7a740c883b06f895a173b5519

    d08eb5b 2018-11-27 Julian Oes - Update opencv includes