Add cgo directives for Darwin arm64 platform via homebrew

[kjg]

I had to add this line to get it to compile successfully on Apple Silicon. It's obviously specific to ssdeep being installed via home-brew, so not sure if this should actually be merged, but there probably aren't a ton of different ways people are installing ssdeep on their macs, so maybe it's an okay default.

At the very least maybe just seeing this can help someone else!