samples/bpf: Move open_raw_sock to separate header

samples/bpf: Move open_raw_sock to separate header

This function was declared in libbpf.c and was the only remaining
function in this library, but has nothing to do with BPF. Shift it out
into a new header, sock_example.h, and include it from the relevant
samples.

Signed-off-by: Joe Stringer <joe@ovn.org>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/20161209024620.31660-8-joe@ovn.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>