Panic due to incorrect usage of winapi

[DomantasJ]

A user on reddit reported that System::new_all() panics on his windows machine. Here's what I found:

1. The panic is caused by this offset_from call because referent is zero sized
2. LPVOID is an alias to *mut c_void
3. Unless std feature is enabled in winapi, it defines c_void as an empty enum

So the use of offset_from does not seem to be valid without enabling winapi/std. Although I'm not sure if winapi is doing the right thing by defining c_void as a zero sized type.

[GuillaumeGomez]

I opened a PR on winapi: retep998/winapi-rs#1003

Let's hope it'll get merged quickly and that a release will follow shortly.

[GuillaumeGomez]

Fixed #571.