redhat: RHEL8 add elfutils-devel as build dependency for kernel module

redhat: RHEL8 add elfutils-devel as build dependency for kernel module

Building the kernel modules under RHEL8 produces the following error
message:
    Makefile:952: *** "Cannot generate ORC metadata for
     CONFIG_UNWINDER_ORC=y, please install libelf-dev, libelf-devel or
     elfutils-libelf-devel".  Stop.

Add elfutils-devel to the BuildRequires in the rpm spec when building
rhel >= 8

Add elfutils-devel to the BuildRequires in the rpm spec that
openafs-kmodtool produces

FIXES 134900

Change-Id: Ie3e03336d9599caa6ceb7879199eab3b12eb971b
Reviewed-on: https://gerrit.openafs.org/13560
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>