rust-bindgen: fix c++ includes
NIX_CXXSTDLIB_COMPILE has been removed in https://github.com/NixOS/nixpkgs/pull/85189
and https://github.com/NixOS/nixpkgs/pull/85189#commitcomment-40987154
remommends using the files in ${clang}/nix-support to find the correct
flags to pass to libclang.