Unvendorize libbpf and build-depend on libbpf-dev

Luca Boccassirequested to merge
bluca/libv4l:libbpf into master

Add libbpf.patch to unvendorize libbpf, which was merged in a very old incarnation when it still lived in the kernel tree, and do minor modifications needed to build with the standalone libbpf.

Closes: #979610

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