clang-lint for PRs isn't checking header files

[JaXt0r]

Currently clang-format is only checking .cc and .hh files. But the API headers are of format .h.

run: 'find include src -iname "*.hh" -or -iname "*.cc" | xargs clang-format --dry-run --style=file --Werror'

I would recommend changing this behavior. But beware: I tested it locally once and it showed some errors on the Header files.