add openssl dev library for cross compilation

[AppleThomas]

#37 566.5 cargo:warning=Could not find directory of OpenSSL installation, and this-sys crate cannot proceed without this knowledge.

[cursor]

PR Summary

Low Risk
Low risk build-only change that adds a missing OpenSSL development dependency; main impact is slightly larger build image and potential build-time dependency resolution differences.

Overview
Fixes sv-dune cross-compilation builds by installing OpenSSL headers/libraries in the build stage (libssl-dev via xx-apt-get) so openssl-sys can compile successfully.

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