fix calling of kpatch-build from git dir

When calling kpatch-build from the git directory, the patch kmod build
fails because it can't find kpatch.h because the symlink is broken.
Copy the kpatch.h file (instead of the symlink) to TEMPDIR.
Merge pull request #22 from jpoimboe/kpatch-build-from-git

fix calling of kpatch-build from git dir