fix cygwin compilation

When compiling under cygwin, access() constants are not defined in fcntl.h:

```
websockify.c: In function ‘main’:
websockify.c:368:35: error: ‘R_OK’ undeclared (first use in this function)
         if (access(settings.cert, R_OK) != 0) {
```

Including unistd.h allows compilation to complete.