Add capstone-coal wings repository as a submodule

[lewismc]

It often happens that while working on one project, you need to use another project from within it. Perhaps it’s a library that a third party developed or that you’re developing separately and using in multiple parent projects. A common issue arises in these scenarios: you want to be able to treat the two projects as separate yet still be able to use one from within the other.
This is exactly what we want to do for using WINGS in COAL-SDS and Git submodules is our mechanism for doing so.