ffi.rs: prevent rustfmt from breaking "use" statements

rustfmt removes the "::" prefix from qualifiers. This breaks the build because
I think a later "pub use ffi::*" results in "std" being an ambiguous reference.

fish-rust/src/ffi.rs

@@ -1,5 +1,7 @@
 use crate::wchar::{self};
+#[rustfmt::skip]
 use ::std::pin::Pin;
+#[rustfmt::skip]
 use ::std::slice;
 use autocxx::prelude::*;
 use cxx::SharedPtr;