Several compile errors in unreal v4.18.3

[chad-williamson]

Need a bit of refactoring due to engine changes...

added
#include "IXRTrackingSystem.h"

Changed
GEngine->HMDDevice.Get();
to
GEngine->XRSystem->GetHMDDevice();

Changed
hmd->SetTrackingOrigin()
to
GEngine->XRSystem->SetTrackingOrigin();

Also, when the game is started, the players vantage point is on the floor, not eye level. Have not been able to figure this one out yet.