/usr/include/osgEarthDrivers/engine_rex/ is in libosgearth-dev 2.9.0+dfsg-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/osgEarthDrivers/engine_rex/Common
/usr/include/osgEarthDrivers/engine_rex/DrawState
/usr/include/osgEarthDrivers/engine_rex/DrawTileCommand
/usr/include/osgEarthDrivers/engine_rex/EngineContext
/usr/include/osgEarthDrivers/engine_rex/GeometryPool
/usr/include/osgEarthDrivers/engine_rex/LayerDrawable
/usr/include/osgEarthDrivers/engine_rex/LoadTileData
/usr/include/osgEarthDrivers/engine_rex/Loader
/usr/include/osgEarthDrivers/engine_rex/MaskGenerator
/usr/include/osgEarthDrivers/engine_rex/RenderBindings
/usr/include/osgEarthDrivers/engine_rex/RexTerrainEngineNode
/usr/include/osgEarthDrivers/engine_rex/RexTerrainEngineOptions
/usr/include/osgEarthDrivers/engine_rex/SelectionInfo
/usr/include/osgEarthDrivers/engine_rex/Shaders
/usr/include/osgEarthDrivers/engine_rex/SurfaceNode
/usr/include/osgEarthDrivers/engine_rex/TerrainCuller
/usr/include/osgEarthDrivers/engine_rex/TerrainRenderData
/usr/include/osgEarthDrivers/engine_rex/TileDrawable
/usr/include/osgEarthDrivers/engine_rex/TileNode
/usr/include/osgEarthDrivers/engine_rex/TileNodeRegistry
/usr/include/osgEarthDrivers/engine_rex/TileRenderModel
/usr/include/osgEarthDrivers/engine_rex/Unloader