Replace libc with rustix in some modules

Unfortunately this isn't a total removal, for two reasons:

- We still need "libc" for the Xlib XIM implementation, for locales.
- BSD requires libc to check for main-threadedness.

First one we can likely resolve in the near future, not so sure about
the second one without using some weird pthreads trick.

src/platform_impl/linux/x11/event_processor.rs

@@ -1,6 +1,11 @@
-use std::{cell::RefCell, collections::HashMap, rc::Rc, slice, sync::Arc};
-
-use libc::{c_char, c_int, c_long, c_ulong};
+use std::{
+    cell::RefCell,
+    collections::HashMap,
+    os::raw::{c_char, c_int, c_long, c_ulong},
+    rc::Rc,
+    slice,
+    sync::Arc,
+};
 
 use x11rb::protocol::xproto::{self, ConnectionExt as _};
 use x11rb::x11_utils::Serialize;