packaging: apply udev hack rule to RHEL

In the RPM spec file there is a test to deploy the uuid hack udev rules for older udev operating systems. This includes CentOS and RHEL, but the check currently only is for CentOS, causing RHEL clients to get a bogus osd rules file.

Adjust the conditional to apply to RHEL as well as CentOS. (The `%{rhel}` macro is defined in both platforms' redhat-rpm-config package.)

Fixes http://tracker.ceph.com/issues/7245