doc/setup.rst: Add BTF kernel config
Add a missing config that appears to be necessary according to: https://patchwork.kernel.org/project/netdevbpf/patch/20211115191840.496263-2-memxor@gmail.com/
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Add a missing config that appears to be necessary according to: https://patchwork.kernel.org/project/netdevbpf/patch/20211115191840.496263-2-memxor@gmail.com/