libcgroup: fix installed-vs-shipped qa issue

When pam is enabled, it complains installed-vs-shipped QA issue:

| ERROR: libcgroup-2.0-r0 do_package: QA Issue: libcgroup:
         Files/directories were installed but not shipped in any package:
|  /lib/security/pam_cgroup.a

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>