How about __linux, or __linux_abi? There are ABIs for other components, and other OSes. Linux is the name of the project after all.
Well I guess you could call it UABI, but that might also imply that it's _userspace_ that defines the interface, instead of the kernel. Since the headers themselves are rather tightly coupled with the kernel, I think I'll stick with the KABI name for now (unless somebody can come up with a better one, of course :-D).