libpam: define limits.conf as CONFFILES of package libpam-runtime

libpam: define limits.conf as CONFFILES of package libpam-runtime

Based as security reason, the system must limit users to
simultaneous system logins, or a site-defined number.
To avoid overwriting the /etc/security/limits.conf file after
upgrading this rpm package, we will define the file as
CONFFILES of package libpam-runtime.

(From OE-Core rev: bdd7c1b088f24e54cf0be83324dd6ffe677af079)

Signed-off-by: Zhixiong Chi <Zhixiong.Chi@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>