rpmspec: do not BuildRequires bpftool on nobuildarches

Herton R. Krzesinskirequested to merge
hertonrk-rh/kernel-ark:nobuildarches-bpftool into os-build 
Kernel builds are failing on i686 due missing or not available bpftool.
It does not make sense to require it on arches that we only build
headers and don't need bpftool, so make it not required on this case.

Signed-off-by: Herton R. Krzesinski <herton@redhat.com>

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