libboringtun.so shared lib

Hi All,

I am trying to build boringtun to use the FFI bindings with C++:

The Build was using cargo build --bin boringtun-cli --release, but when I try to import:

1: /lib64/libboringtun.so: undefined symbol: x25519_secret_key
2: /lib64/libboringtun.so: undefined symbol: x25519_public_key
3: /lib64/libboringtun.so: undefined symbol: x25519_key_to_base64
4: /lib64/libboringtun.so: undefined symbol: x25519_key_to_str_free

Start with the reported `cargo build --bin boringtun-cli --release` command and the FFI binding entry points, then inspect how the shared library exposes the four reported x25519 symbols when loaded from C++. Done means a generated `libboringtun.so` resolves all four symbols without undefined-symbol errors.

Written by the indexing model from the issue text.