get_fedora_kheaders: allow getting older kernel headers

With Fedora, especially with rawhide, it's entirely possible for kernel to be built and an older kernel-headers package being used for it. This patch allows the fetcher to download a kernel-headers package that was built before the kernel regardless of its version, if it's not possible to get an exact match. This may all require mork tweaking though.

Signed-off-by: Jakub Racek jracek@redhat.com