libbpf: install the header file libbpf.h

It seems like an oversight not to install the header file for libbpf,
given the libbpf.so + libbpf.a files are installed.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>