/usr/include/Box2D/Dynamics/Joints/ is in libbox2d-dev 2.3.1+ds-3.
This file is owned by root:root, with mode 0o755.
..
/usr/include/Box2D/Dynamics/Joints/b2DistanceJoint.h
/usr/include/Box2D/Dynamics/Joints/b2FrictionJoint.h
/usr/include/Box2D/Dynamics/Joints/b2GearJoint.h
/usr/include/Box2D/Dynamics/Joints/b2Joint.h
/usr/include/Box2D/Dynamics/Joints/b2MotorJoint.h
/usr/include/Box2D/Dynamics/Joints/b2MouseJoint.h
/usr/include/Box2D/Dynamics/Joints/b2PrismaticJoint.h
/usr/include/Box2D/Dynamics/Joints/b2PulleyJoint.h
/usr/include/Box2D/Dynamics/Joints/b2RevoluteJoint.h
/usr/include/Box2D/Dynamics/Joints/b2RopeJoint.h
/usr/include/Box2D/Dynamics/Joints/b2WeldJoint.h
/usr/include/Box2D/Dynamics/Joints/b2WheelJoint.h