Move SimGear to current stable release 2018.3.5
Move SimGear to latest stable release (apparently, 2019.1.1 was a development release, now unsupported).
No specific changelog for SimGear.
Test Plan: Built FlightGear 2018.3.5 agaisnt this version and tested it.
Reviewers: Triage Team, DataDrake
Reviewed By: Triage Team, DataDrake
Differential Revision: https://dev.getsol.us/D8891