Fix validate_tokens for multiple EOS tokens (#347)

`validate_tokens` in `parser/src/earley/parser.rs` only recognized the
primary EOS token, which was overlooked in #305 when adding support for
multiple EOS tokens.

This PR aligns the behaviour with e.g. `compute_mask` that now allows
multiple eos tokens.

Signed-off-by: Dennis Keck <26092524+fellhorn@users.noreply.github.com>