rand: fix libc fallout

Author: emberian

src/librand/os.rs

@@ -59,9 +59,11 @@ mod imp {
 
 #[cfg(windows)]
 mod imp {
+    extern crate libc;
+
     use Rng;
     use std::cast;
-    use std::libc::{c_ulong, DWORD, BYTE, LPCSTR, BOOL};
+    use self::libc::{c_ulong, DWORD, BYTE, LPCSTR, BOOL};
     use std::os;
     use std::rt::stack;