/usr/include/ql/termstructures/volatility/optionlet/ is in libquantlib0-dev 1.7.1-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/ql/termstructures/volatility/optionlet/all.hpp
/usr/include/ql/termstructures/volatility/optionlet/capletvariancecurve.hpp
/usr/include/ql/termstructures/volatility/optionlet/constantoptionletvol.hpp
/usr/include/ql/termstructures/volatility/optionlet/optionletstripper.hpp
/usr/include/ql/termstructures/volatility/optionlet/optionletstripper1.hpp
/usr/include/ql/termstructures/volatility/optionlet/optionletstripper2.hpp
/usr/include/ql/termstructures/volatility/optionlet/optionletvolatilitystructure.hpp
/usr/include/ql/termstructures/volatility/optionlet/spreadedoptionletvol.hpp
/usr/include/ql/termstructures/volatility/optionlet/strippedoptionlet.hpp
/usr/include/ql/termstructures/volatility/optionlet/strippedoptionletadapter.hpp
/usr/include/ql/termstructures/volatility/optionlet/strippedoptionletbase.hpp