libssh2-sys: build fix required

[nielx]

Since Rust 1.58.0 (and thus cargo 0.59.0) there is an issue linking everything together. The libcurl.so dependency has undefined symbols that are expected to be in libssh2.so, but since we have a set of static symbols but hidden symbols in the rust libssh2-sys crate, the linker complains that there are undefined symbols in a DSO referring to the hidden symbol in libssh2-sys.rlib.

There is a fix for the libss2-sys crate that needs to be upstreamed.

[nielx]

Upstream PR: https://github.com/alexcrichton/ssh2-rs/pull/242