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

Signed-off-by: Ken Dreyer <ken.dreyer@inktank.com>
(cherry picked from commit 64a0b4fa563795bc22753940aa3a4a2946113109)