Win32: fixed wrong type cast.

GetQueuedCompletionStatus() document on MSDN says the
following signature:

    http://msdn.microsoft.com/en-us/library/windows/desktop/aa364986.aspx

    BOOL WINAPI GetQueuedCompletionStatus(
      _In_   HANDLE CompletionPort,
      _Out_  LPDWORD lpNumberOfBytes,
      _Out_  PULONG_PTR lpCompletionKey,
      _Out_  LPOVERLAPPED *lpOverlapped,
      _In_   DWORD dwMilliseconds
    );

In the latest specification, the type of the third argument
(lpCompletionKey) is PULONG_PTR not LPDWORD.
Win32: suppressed warnings by "-Werror=sign-compare".