test: rbitcoin-net tests don't build on macOS #1182

On a Mac, cargo test --workspace stops with a build error before any tests run:

error[E0433]: cannot find module or crate `libc` in this scope
   --> crates/rbitcoin-net/tests/i2p_sam.rs:197

i2p_sam.rs uses libc on every unix system, but rbitcoin-net/Cargo.toml only adds libc on Linux. So on macOS the test can't find it.

CI doesn't catch this because it runs on Linux. It only hits people running the tests on their own Mac. They have to skip rbitcoin-net to run the rest of the suite.

Fix:

Add libc to rbitcoin-net/Cargo.toml for all unix systems, not just Linux. Or only use libc in i2p_sam.rs when running on Linux.