security: only build lsm_audit if CONFIG_SECURITY=y

The lsm_audit code is only required when CONFIG_SECURITY is enabled.
It does not have a build dependency on CONFIG_AUDIT since audit.h
provides trivial static inlines for audit_log*() when CONFIG_AUDIT
is disabled.  Hence, the Makefile should only add lsm_audit to the
obj lists based on CONFIG_SECURITY, not CONFIG_AUDIT.

Fixes: 59438b46 ("security,lockdown,selinux: implement SELinux lockdown")
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>