/usr/include/spatialindex/capi/ is in libspatialindex-dev 1.8.1-3.
This file is owned by root:root, with mode 0o755.
..
/usr/include/spatialindex/capi/BoundsQuery.h
/usr/include/spatialindex/capi/CountVisitor.h
/usr/include/spatialindex/capi/CustomStorage.h
/usr/include/spatialindex/capi/DataStream.h
/usr/include/spatialindex/capi/Error.h
/usr/include/spatialindex/capi/IdVisitor.h
/usr/include/spatialindex/capi/Index.h
/usr/include/spatialindex/capi/LeafQuery.h
/usr/include/spatialindex/capi/ObjVisitor.h
/usr/include/spatialindex/capi/Utility.h
/usr/include/spatialindex/capi/sidx_api.h
/usr/include/spatialindex/capi/sidx_config.h
/usr/include/spatialindex/capi/sidx_impl.h