Update submodule sitl_gazebo to latest Tue Aug 21 01:37:53 UTC 2018

    - sitl_gazebo in PX4/Firmware (7f41ead2): https://github.com/PX4/sitl_gazebo/commit/c1ca87e37b831cd4b64ac642957e701875738909
    - sitl_gazebo current upstream: https://github.com/PX4/sitl_gazebo/commit/821c55ca714d7acf93d8ed96af71237b8437e488
    - Changes: https://github.com/PX4/sitl_gazebo/compare/c1ca87e37b831cd4b64ac642957e701875738909...821c55ca714d7acf93d8ed96af71237b8437e488

    821c55c 2018-08-20 Elia Tarasov - remove unused destination socket address
37faf25 2018-08-20 Elia Tarasov - remove undecalred member function
d5ca1d4 2018-08-20 Elia Tarasov - fix default QGC udp port initialization

sitl_gazebo @ 821c55ca

-Subproject commit c1ca87e37b831cd4b64ac642957e701875738909
+Subproject commit 821c55ca714d7acf93d8ed96af71237b8437e488