kdl_typekit: disabled tests if CATKIN_ENABLE_TESTING is false

CMake Error at /opt/ros/hydro/share/catkin/cmake/test/tests.cmake:17 (message):
  catkin_add_gtest() is not available when tests are not enabled.  The CMake
  code should only use it inside a conditional block which checks that
  testing is enabled:

  if(CATKIN_ENABLE_TESTING)

    catkin_add_gtest(...)

  endif()

Call Stack (most recent call first):
  test/CMakeLists.txt:7 (catkin_add_gtest)