Update submodule sitl_gazebo to latest Wed Feb 17 12:42:47 UTC 2021 (#…

…16862)

    - sitl_gazebo in PX4/Firmware (51dd141): PX4/PX4-SITL_gazebo-classic@4f26815
    - sitl_gazebo current upstream: PX4/PX4-SITL_gazebo-classic@bebb9a9
    - Changes: PX4/PX4-SITL_gazebo-classic@4f26815...bebb9a9

bebb9a9 2021-02-11 Dongoo Lee - Add depth camera model for ROS2 environment (#707)
54e1a19 2021-02-10 TSC21 - gazebo_mavlink_interface: generate subscribers for nested models with sensors
97c167d 2021-02-10 Julian Oes - Implement gimbal controller as MAVLink gimbal v2 device (#703)

Co-authored-by: PX4 BuildBot <bot@px4.io>