include: dual license the bpf helper includes

include: dual license the bpf helper includes

The files bpf_api.h and bpf_elf.h are useful for TC BPF programs
to use. And there is no requirement that those be GPL only;
we intend to allow BSD licensed BPF helpers as well.

This makes the file license same as libbpf.

Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>