postfix check warm me that some file differ

/var/spool/postfix is a chroot where postfix can optionally run, The idea is that if postfix is somehow compromised, the only thing the attacker would have access to is this small subset of your system instead of your whole system. The warning here is that the files in /var/spool/postfix no longer match the files in your regular system that they were copied from. You might verify that the files in /lib were expected to change (like, was libnss recently upgraded?), then consider copying these versions into the /var/spool/postfix/ chroot, so that the chroot also gets this upgrade.

As per stew's answer, to kindly save people typing. As root:

cd /lib; cp libnss_files-2.11.3.so libnss_nis-2.11.3.so libnss_dns-2.11.3.so libnss_compat-2.11.3.so libnss_nisplus-2.11.3.so libnss_hesiod-2.11.3.so /var/spool/postfix/lib/