Only depend on kernel32 and winapi on Windows

[chris-morgan]

kernel32 and winapi are completely pointless for non-Windows builds. (They’re empty crates on non-Windows owing to a #![cfg(windows)] crate attribute so that they build.) They shouldn’t even be depended on for non-Windows builds.

The duplication is ugly, but necessary for now; see also rust-lang/cargo#1007

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[alexcrichton]

For now we're idiomatically just adding these dependencies to avoid the duplication across all our crates. For example the rand crate also depends on these two crates. Build times shouldn't be affected by this very much at all and in general it seems like there aren't enough cons to warrant this for now. We can certainly do so once the associated Cargo issue is fixed!