silence a warning about PhantomData in repr(C) structs

Since PhantomData is a ZST, it wasn't considered safe to use in a
repr(C) struct. In practice, however, since the C side doesn't care
about the PhantomData, it's fine. Also this particular case no longer
causes the warning on nightlies:
rust-lang/rust-bindgen#442

rust_src/src/lib.rs

@@ -7,6 +7,7 @@
 #![allow(non_snake_case)]
 #![allow(private_no_mangle_fns)]
 #![allow(non_camel_case_types, non_snake_case, non_upper_case_globals)]
+#![allow(improper_ctypes)] // we need this to be able to inclde FieldOffsets in C structs
 #![feature(proc_macro)]
 #![cfg_attr(feature = "strict", deny(warnings))]
 #![feature(global_allocator)]