I have a large codebase licensed under GPL. There is another codebase I found one specific file to be useful in, and the whole codebase is under Apache 2. I cannot change the license of my project, nor include their entire project as a submodule, since it is very large. Additionally, I will be modifying the code from that file significantly. How do I approach adding this to my project. Is it sufficient just to leave the copyright header (that also specifies the license)? Or do I need to also include their license file somewhere?