audit: hide auditsc_get_stamp and audit_serial prototypes

auditsc_get_stamp() and audit_serial() are internal audit functions so
move their prototypes from include/linux/audit.h to kernel/audit.h
so they are not visible to the rest of the kernel.

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>