Fix compile on nightly

[alexcrichton]

The lints for C apis were tweaked recently and 0-size structs aren't liked.
These should be empty enums anyway, however, to ensure they're never created.

[rust-highfive]

r? @huonw

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[tamird]

This technique probably deserves a comment. It suggests that the solution in rust-lang/rust#27267 and some of the fallout fixes in rust-lang/rust#26583 were incorrect (changing from 0-size structs to type aliases of c_void).

[alexcrichton]

@bors: r+ 60e66b3 (now that I see travis passing)

@tamird it kinda just depends what you want to do in each case. I personally prefer to have empty enums as it introduces a new type that's not compatible with c_void (e.g. type is just an alias), but they're both legitimate possibilities.

[bors]

⌛ Testing commit 60e66b3 with merge 083fa7c...

[tamird]

cc rust-lang/rust#27303

[bors]

☀️ Test successful - cargo-linux-32, cargo-linux-64, cargo-mac-32, cargo-mac-64, cargo-win-gnu-32, cargo-win-gnu-64, cargo-win-msvc-32, cargo-win-msvc-64