To help catch ABI breaks at compile-time, add compile-time assertions to
verify the siginfo_t layout.

Link: https://lkml.kernel.org/r/20210505141101.11519-3-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/20210429190734.624918-3-elver@google.com
Link: https://lkml.kernel.org/r/87sg0fxx92.fsf_-_@disp2133Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>