Dependencies for "external LLVM"

[rene-d]

The instructions to use https://github.com/aya-rs/bpf-linker#using-external-llvm are incomplete.

Indeed, in order to compile bpf-linker, I also had to install libpolly-15-dev from https://apt.llvm.org and zlib1g-dev on a Ubuntu 22.04.1 aarch64 virtual machine (on Apple Silicon).

After that, everything is fine and I can test some eBPF programs :)