libbpf: install linux/btf.h
libbpf: install linux/btf.h

This installs the linux/btf.h header in libbpf, which is necessarily
compatible with bpf/btf.h
Note that we also ship the linux header's linux/btf.h with glibc,
but explicit buildInputs should have priority over default headers so
hopefully this one will always be used.

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