Fix missing compatibility header for Wily

[davetcoleman]

Wily was still failing when building tests:

Errors     << moveit_core:make /root/ws_moveit/logs/moveit_core/build.make.001.log
In file included from /root/ws_moveit/src/moveit/moveit_core/constraint_samplers/test/pr2_arm_ik.cpp:39:0:
/root/ws_moveit/src/moveit/moveit_core/constraint_samplers/test/pr2_arm_ik.h:170:34: error: ‘**urdf::JointConstSharedPtr**’ has not been declared
   void addJointToChainInfo(urdf::JointConstSharedPtr joint, moveit_msgs::KinematicSolverInfo &info);
                                  ^
/root/ws_moveit/src/moveit/moveit_core/constraint_samplers/test/pr2_arm_ik.cpp: In member function ‘bool pr2_arm_kinematics::PR2ArmIK::init(const urdf::ModelInterface&, const string&, const string&)’:
/root/ws_moveit/src/moveit/moveit_core/constraint_samplers/test/pr2_arm_ik.cpp:61:3: error: ‘**LinkConstSharedPtr**’ is not a member of ‘urdf’
   urdf::LinkConstSharedPtr link = robot_model.getLink(tip_name);

Need to include changes in ros/robot_model#160

[davetcoleman]

Travis passed! This is what is breaking https://travis-ci.org/ros-planning/moveit_ci/jobs/176416385 and I think what is blocking Kinetic

[130s]

Wow, that makes the simplest sense!