Fix warning "redirecting incorrect #include <sys/poll.h> to <poll.h>"

This warning occurs on Alpine with musl:

```
In file included from usb.cpp:22:
/usr/include/sys/poll.h:1:2: warning: #warning redirecting incorrect #include <sys/poll.h> to <poll.h> [-Wcpp]
    1 | #warning redirecting incorrect #include <sys/poll.h> to <poll.h>
      |  ^~~~~~~
```

src/usb/usb.cpp

@@ -19,7 +19,7 @@
 
 #include <cstdlib>
 #include <cerrno>
-#include <sys/poll.h>
+#include <poll.h>
 #include "usb.h"
 #include "types.h"