Fix compilation with --cfg doc

[Thomasdezeeuw]

Fixes #383

Does this fix your issue @fpoli?

[fpoli]

Yes, thank you. With this revision cargo doc works fine. What I used to test is:

mkdir new-test-crate
cd new-test-crate
cargo init
cargo add --git https://github.com/rust-lang/socket2.git --rev 7b1e1ad841ffdb754ce534dc817c749d8211cc1d
RUSTDOCFLAGS="--generate-link-to-definition -Zunstable-options" cargo +nightly doc

Without the --rev 7b1e1ad841ffdb754ce534dc817c749d8211cc1d the last command fails, as reported in #383.