backends: Don't depend on linux/bpf.h to compile bpf disassembler.

backends: Don't depend on linux/bpf.h to compile bpf disassembler.

We only need a few constants and one structure definition from linux/bpf.
Just define those in a local lib/bpf.h file. This makes sure the bpf
disassembler is always build and included even when elfutils is build
on older GNU/Linux systems (and even on other platforms).

Signed-off-by: Mark Wielaard <mark@klomp.org>