tools: bpf: fix license for a compat header file

libc_compat.h is used by libbpf so make sure it's licensed under
LGPL or BSD license.  The license change should be OK, I'm the only
author of the file.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>