fix-bindgen-compiler-includes-20221018_01

add missing include directory for bindgen's clang

Bindgen uses clang to preprocess, parse and type check C/C++ header
files (https://rust-lang.github.io/rust-bindgen/requirements.html).
To ensure hermiticity, rules_rust dowloads a prebuilt clang binary and
registers it as a custom toolchain for invoking bindgen. However, it
fails to register the toolchain provided include directory.

This was causing errors on systems that did not have clang installed.
This patches rules_rust to register this include path.

Signed-off-by: Timothy Trippel <ttrippel@google.com>