Silence long-ignored warnings about Drop for C types.

src/lib.rs

@@ -10,6 +10,7 @@
 #![feature(unsafe_no_drop_flag)]
 #![feature(const_fn)]
 
+#![allow(drop_with_repr_extern)]
 #![allow(non_upper_case_globals, non_camel_case_types, non_snake_case, improper_ctypes)]
 
 #[macro_use]