Go to Engine\Source\Runtime\Engine\Public\AI\Navigation\RecastNavMeshGenerator.h
and change this line of code:
struct FRecastGeometryCache
to
struct ENGINE_API FRecastGeometryCache
then recompile whole the engine code and You should be able to use FRecastGeometryCache::FRecastGeometryCache in Your module, but if you can’t compile the engine code, you should wait for EPIC guys or some good samaritan who will make pull request with this piece of code
Regards