bpf: Selftests, move sockmap bpf prog header into progs

Moves test_sockmap_kern.h into progs directory but does not change
code at all.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/158939718921.15176.5766299102332077086.stgit@john-Precision-5820-Tower