Set duplicate targets to be built for gymkhana

[onurbagoren]

Was getting the error

CMake Deprecation Warning at vorc/vorc_gazebo/CMakeLists.txt:13 (cmake_policy):
  The OLD behavior for policy CMP0002 will be removed from a future version
  of CMake.

  The cmake-policies(7) manual explains that the OLD behaviors of all
  policies are deprecated and that a policy should be set to OLD only under
  specific short-term circumstances.  Projects should be ported to the NEW
  behavior and not rely on setting a policy to OLD.

when running colcon build. Added to the CMakeLists.txt the flag to set duplicate targets, as it appeared that there were multiple targets of gymkhana being built.