bpf: install libbpf headers on 'make install'

Add a new target to install the bpf.h header to $(prefix)/include/bpf/
directory.  This is necessary to build standalone applications using
libbpf, without the need to clone the kernel sources and point to them.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>