kexec-tools: add mirror://kernel URI

From http://horms.net/projects/kexec/kexec-tools:

  "The canonical site for kexec-tools release is
   http://kernel.org/pub/linux/utils/kernel/kexec/. It should also be
   available on the extensive network of kernel.org mirror sites and
   it is mirrored at http://horms.net/projects/kexec/kexec-tools/
   just for good measure."