include hidapi sources in tree

Hi Geert!

I ended up with including HIDAPI sources directly in my copy of go.hid. This allows Go to leverage conditional compilation because of the various suffixes of source files. This means that "go build" will work without any problems on every platform (Windows, Linux, Mac) out of the box, without the need for any manual steps (which are different for each platform).

I thought you might be interested in this change so I created a pull request for you.