sitl_gazebo: update submodule

This should fix a race because the use of future/promise wasn't done in
a thread-safe way.

sitl_gazebo @ 4cb3a03b

-Subproject commit d27efd7937be8dc8453abc41be641a6660ecdc13
+Subproject commit 4cb3a03b472becf8aa89c2e747a49e4f2f593676