Include necessay header file

Fixes #4 `socks.h` contains some data type from `<sys/select>`.

 I think `gcc` may do some tricks causing ignoring the order of include headers while `clang` does not.
So when compiling `socks.c`, it works fine for `gcc`
```C 
#include "socks.h" // need data types from <sys/select.h>
#include "socket_wrap.h"  // already includes <sys/select.h>
```