Overriding <linux/audit.h> to implicitly include <libaudit.h> turns out to
be problematic. Because <linux/audit.h> is pulled in by various other kernel
headers, any source file including those kernel headers indirectly ends up
depending on <libaudit.h>, even if the executable or library being built
does not require libaudit at all.
Let's drop the override header and include <libaudit.h> only where explicitly
needed. This also moves the fallback definitions of AUDIT_SERVICE_* and
MAX_AUDIT_MESSAGE_LENGTH to libaudit-util.h.