std::rt::io: Fix file I/O on Win32

It was broken on win32 because of header inconsistency.

Author: klutzy

src/libstd/rt/uv/uvll.rs

@@ -67,11 +67,20 @@ pub mod errors {
     pub static EPIPE: c_int = -libc::EPIPE;
 }
 
+// see libuv/include/uv-unix.h
+#[cfg(unix)]
 pub struct uv_buf_t {
     base: *u8,
     len: libc::size_t,
 }
 
+// see libuv/include/uv-win.h
+#[cfg(windows)]
+pub struct uv_buf_t {
+    len: u32,
+    base: *u8,
+}
+
 pub type uv_handle_t = c_void;
 pub type uv_loop_t = c_void;
 pub type uv_idle_t = c_void;