/usr/include/osgEarthDrivers/engine_osgterrain/ is in libosgearth-dev 2.5.0+dfsg-8build1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/osgEarthDrivers/engine_osgterrain/Common
/usr/include/osgEarthDrivers/engine_osgterrain/CustomTerrainTechnique
/usr/include/osgEarthDrivers/engine_osgterrain/DynamicLODScaleCallback
/usr/include/osgEarthDrivers/engine_osgterrain/FileLocationCallback
/usr/include/osgEarthDrivers/engine_osgterrain/KeyNodeFactory
/usr/include/osgEarthDrivers/engine_osgterrain/LODFactorCallback
/usr/include/osgEarthDrivers/engine_osgterrain/MultiPassTerrainTechnique
/usr/include/osgEarthDrivers/engine_osgterrain/OSGTerrainEngineNode
/usr/include/osgEarthDrivers/engine_osgterrain/OSGTerrainOptions
/usr/include/osgEarthDrivers/engine_osgterrain/OSGTileFactory
/usr/include/osgEarthDrivers/engine_osgterrain/ParallelKeyNodeFactory
/usr/include/osgEarthDrivers/engine_osgterrain/SerialKeyNodeFactory
/usr/include/osgEarthDrivers/engine_osgterrain/SinglePassTerrainTechnique
/usr/include/osgEarthDrivers/engine_osgterrain/StreamingTerrainNode
/usr/include/osgEarthDrivers/engine_osgterrain/StreamingTile
/usr/include/osgEarthDrivers/engine_osgterrain/TerrainNode
/usr/include/osgEarthDrivers/engine_osgterrain/Tile
/usr/include/osgEarthDrivers/engine_osgterrain/TileBuilder
/usr/include/osgEarthDrivers/engine_osgterrain/TransparentLayer