When the build is performed with cmake option `-DPOLLER=select`, the following warning is triggered from 4 locations that import `polling_util.hpp`:
```
[...]/src/polling_util.hpp:115:50: warning: unused parameter 'pollset_' [-Wunused-parameter]
inline size_t valid_pollset_bytes (const fd_set &pollset_)
^
1 warning generated.
```
This is fixed here.