docs: mention that libclang is required

docs: mention that libclang is required

This fixes the following build failure on Debian bookworm:

    error: failed to run custom build command for `clang-sys v1.6.1`

    Caused by:
      process didn't exit successfully: `/home/minoru/src/bcachefs-tools/rust-src/target/release/build/clang-sys-df95f6d1266be773/build-script-build` (exit status: 101)
      --- stdout
      cargo:warning=could not execute `llvm-config` one or more times, if the LLVM_CONFIG_PATH environment variable is set to a full path to valid `llvm-config` executable it will be used to try to find an instance of `libclang` on your system: "couldn't execute `llvm-config --prefix` (path=llvm-config) (error: No such file or directory (os error 2))"

      --- stderr
      thread 'main' panicked at /home/minoru/.cargo/registry/src/index.crates.io-6f17d22bba15001f/clang-sys-1.6.1/build/dynamic.rs:206:45:
      called `Result::unwrap()` on an `Err` value: "couldn't find any valid shared libraries matching: ['libclang.so', 'libclang-*.so'], set the `LIBCLANG_PATH` environment variable to a path where one of these files can be found (invalid: [])"
      note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>