Fix Warnings

````
lib/threadpool.c:96:71: error: unused parameter 'flags' [-Werror,-Wunused-parameter]
threadpool_t *threadpool_create(int thread_count, int queue_size, int flags)
                                                                      ^
lib/threadpool.c:147:40: error: unused parameter 'flags' [-Werror,-Wunused-parameter]
                   void *argument, int flags)
                                       ^
2 errors generated.
````