RPM require perl(XML::Simple) explicitly

On RHEL 8 node.

```
Can't locate XML/Simple.pm in @INC (you may need to install the
XML::Simple module) (@INC contains: /usr/local/lib64/perl5
/usr/local/share/perl5 /usr/lib64/perl5/vendor_perl
/usr/share/perl5/vendor_perl /usr/lib64/perl5 /usr/share/perl5) at
/usr/sbin/cern-get-keytab line 60.
BEGIN failed--compilation aborted at /usr/sbin/cern-get-keytab line 60.
```

after installing perl(XML::Simple) all good.

```
Waiting for password replication (0 seconds past)
Waiting for password replication (5 seconds past)
Waiting for password replication (10 seconds past)
Keytab file saved: /etc/krb5.keytab
```
13 jobs for xmlsimple in 3 minutes and 51 seconds (queued for 3 seconds)