bump bindgen back to 0.54 for compatibility with rocksdb

[yaahc]

Prior to this PR, adding a rocksdb dependency to zebra-state causes a dependency resolver error in cargo, because we end up depending on two different versions of clang-sys which can only be linked in once per application.

This change decrements the bindgen version we use from 0.55 to the version used by rocksdb, 0.54, which causes the dependency resolver to find a single version of clang-sys which works for both dependencies.