libc: re-export libc properly

With rust 1.7, the following warning was being emitted by the compiler:

    warning: `pub extern crate` does not work as expected and should
    not be used. Likely to become an error. Prefer `extern crate` and
    `pub use`.

Based on rust-lang/rust#26775 it appears that
this change is the current best approach for properly exporting the
libc dependency so it may be used outside of nix.

Signed-off-by: Paul Osborne <osbpau@gmail.com>

src/lib.rs

@@ -13,12 +13,15 @@
 #[macro_use]
 extern crate bitflags;
 
-pub extern crate libc;
-
 #[cfg(test)]
 extern crate nix_test as nixtest;
 
+mod exports {
+    extern crate libc;
+}
+
 // Re-exports
+pub use exports::libc;
 pub use libc::{c_int, c_void};
 pub use errno::Errno;