Fix vendored-openssl feature on macOS

[alexcrichton]

Turns out the git2 dependency on openssl-sys was conditional to happen
everywhere but macOS, which means that vendored-openssl didn't work. The
fix here is to forward the feature to the libgit2-sys crate which
depends on openssl-sys even on macOS.

Closes #718

[ehuss]

Is there a particular reason that openssl-sys is excluded from macos here?

git2-rs/Cargo.toml

Line 25 in b055a28

[target."cfg(all(unix, not(target_os = \"macos\")))".dependencies]

libgit2-sys does not have the same macos exclusion, so I believe it has no effect to have it.

[alexcrichton]

Hm at this point it's probably historical for one reason or another, I don't recall the original reason but I would agree that it probably doesn't need to be present any more. (that being said it seemed fine to go ahead and add a vendored-openssl feature to libgit2-sys as well)