libbpf: move logging helpers into libbpf_internal.h

libbpf_util.h header was recently exposed as public as a dependency of
xsk.h. In addition to memory barriers, it contained logging helpers,
which are not supposed to be exposed. This patch moves those into
libbpf_internal.h, which is kept as an internal header.

Cc: Stanislav Fomichev <sdf@google.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Fixes: 7080da89 ("libbpf: add libbpf_util.h to header install.")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>