changed to include <poll.h> instead of <sys/poll.h>

This is a very cosmetic change but I think it is good to be standards compliant.

The standard defines ```<poll.h>``` and not ```<sys/poll.h>```.

http://pubs.opengroup.org/onlinepubs/009696799/basedefs/poll.h.html

When building against musl libc it silences some annoying cpp warnings like this:
```
warning: #warning redirecting incorrect #include <sys/poll.h> to <poll.h> [-Wcpp]
```