Only build BPF code on x86_64 and i686
    
    Currently (v4l-utils version 1.16.2), trying to build the BPF
    decoders failed for non-x86 archs. While this is not solved,
    only enable building it on x86, as this is an optional feature
    anyway.
    
    Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>