Ubuntu 22.04 installation requirements added

[Yeaseen]

For c2rust to be installed on Ubuntu 22.04, the library libstdc++-12-dev is a prerequisite. I added this to the Readme file. Also, I discarded any .DS_store file to be uploaded on 'GitHub' in the .gitignore file.

[fw-immunant]

Hi, what error do you observe when installing with the current (sans libstdc++12) set of dependencies? libstdc++12 is not a direct dependency of c2rust, so it doesn't make sense to install it explicitly. I believe this is an upstream packaging error, which seems to be tracked here: https://bugs.launchpad.net/ubuntu/+source/llvm-toolchain-14/+bug/2038340