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 am taking the opportunity to fix a couple of warnings about an unneeded mut and an unused import.