Fix Windows build

I was getting this error:

error[E0609]: no field `fd` on type `*mut glib_sys::GPollFD`
  --> gstreamer\src\bus_windows.rs:42:20
   |
42 |             pollfd.fd as *mut _
   |             -------^^
   |             |
   |             help: `pollfd` is a raw pointer; try dereferencing it: `(*pollfd).fd`

I take the opportunity to remove a couple of warnings about an unused import and an unneeded mut.