audit: remove duplicate export of audit_enabled

audit_enabled has already been exported in
include/linux/audit.h. and kernel/audit.h
includes include/linux/audit.h, no need to
export aduit_enabled again in kernel/audit.h

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Eric Paris <eparis@redhat.com>