rust-xgboost from our fork in postgresml/rust-xgboost doesn't build on Mac OS since the migration to v2.0.

To reproduce on a Mac:

1. Checkout the repo postgresml/rust-xgboost
2. git submodule update --init --recursive
3. cargo build

An error similar to:

    |
365 |     pub static std_value: _Tp;
    |                           ^^^ not found in this scope

will appear.

Reverting to commit 3d4bd10b70117b94367d3c300d233252a204061d fixes the compilation issue.