An issue on calling luaL_get_ctypeid()

In the pull request #89 , when I tried to call ``luaL_get_ctypeid()`` on ``struct gk_config *`` and ``struct gk_fib_dump_entry *``, both return values are the same (i.e., 166) even they are called at the same Lua state. However, they are expected to be different, but it is up to the LuaJIT to decide since ``luaL_get_ctypeid()`` literally calls LuaJIT to find out. The relevance of using the same Lua state is that these IDs are generated dynamically, so each added type will create a new ID.