/usr/include/bullet/BulletDynamics/ConstraintSolver/ is in libbullet-dev 2.87+dfsg-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/bullet/BulletDynamics/ConstraintSolver/btConeTwistConstraint.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btConstraintSolver.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btContactConstraint.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btContactSolverInfo.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btFixedConstraint.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btGearConstraint.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btGeneric6DofConstraint.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btGeneric6DofSpring2Constraint.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btGeneric6DofSpringConstraint.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btHinge2Constraint.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btHingeConstraint.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btJacobianEntry.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btNNCGConstraintSolver.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btPoint2PointConstraint.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btSequentialImpulseConstraintSolver.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btSliderConstraint.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btSolve2LinearConstraint.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btSolverBody.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btSolverConstraint.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btTypedConstraint.h
/usr/include/bullet/BulletDynamics/ConstraintSolver/btUniversalConstraint.h