libpam: Fix the xtests/tst-pam_motd[1|3] failures

Reproducer:
1.Enable the ptest of libpam and build the image.
2.Boot the rootfs with nfs, then run the following tests as root:
 cd /usr/share/Linux-PAM/xtests
 /usr/share/Linux-PAM/xtests# ./run-xtests.sh . tst-pam_motd1
 /usr/share/Linux-PAM/xtests# ./run-xtests.sh . tst-pam_motd3

After applying this patch, the ptest doesn't be failed.

Signed-off-by: Zhixiong Chi <zhixiong.chi@windriver.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>