[ecash_lib_wasm] Exclude from the workspace

Summary:
This doesn't cross build, exclude it from the workspace so it's not part
of the Cargo.lock and not part of the default cargo build.

Test Plan:

./contrib/teamcity/build-configurations.py ecash-lib-tests

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15943