Fix E0512 error

[uchiumih]

Prior to this fix, when I ran cargo check I got the following error.

% cargo check
error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
   --> /Users/worker/.cargo/registry/src/jiasu.xzqcsaa.nyc.mn-1ecc6299db9ec823/socket2-0.3.12/src/sockaddr.rs:176:9
    |
176 |         mem::transmute::<SocketAddrV4, sockaddr_in>(v4);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: source type: `SocketAddrV4` (48 bits)
    = note: target type: `sockaddr_in` (128 bits)

For more information about this error, try `rustc --explain E0512`.
error: could not compile `socket2` due to previous error
warning: build failed, waiting for other jobs to finish...

[uchiumih]

I created a new PR #13 and will close it.