l2md: get l2md to compile and run on MacOS

Few tweaks to make the code compile on MacOS. As a prerequisite to get
it running, `brew install libgit2` is needed.

Signed-off-by: Alexei Starovoitov <ast@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
6 files changed