audit.h: replace defines with C stubs

Replace the #defines used when CONFIG_AUDIT or CONFIG_AUDIT_SYSCALLS are
disabled so we get type checking during those builds.

Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>