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/mod.rs

@@ -230,7 +230,7 @@ impl<T: 'static> EventLoop<T> {
                 xconn.display,
                 &mut xinput_major_ver,
                 &mut xinput_minor_ver,
-            ) != ffi::Success as libc::c_int
+            ) != ffi::Success as std::os::raw::c_int
             {
                 panic!(
                     "X server has XInput extension {xinput_major_ver}.{xinput_minor_ver} but does not support XInput2",