set spool_dir seltype to syslogd_var_lib_t

This type is necessary for rsyslogd to be able to write in spool_dir
when selinux is enabled and enforcing. For some reason, this directory
does not have the correct seltype in RHEL <= 7 and using disk queues or
.state files does not work.
Merge pull request #98 from ckaenzig/spool-dir-seltype

set spool_dir seltype to syslogd_var_lib_t