Add this after the call to add_library
set_target_properties(gazebo_ros_moveit_planning_scene PROPERTIES LINKER_LANGUAGE CXX)

Side note, Is all the code for your gazebo plugin in gazebo_ros_moveit_planning_scene.h? Often code is divided into a header and source file. If all the code is in that file then it is more common to name it .cpp or .cc. The extension .h is usually given to C header files.