Fix conditional logic for systemd/systemd-udev split

The `0%{?rhel} <= 7` conditional is incorrect.  It evaluates true on
Fedora, but we want to require systemd-udev there.  It also is incorrect
for RHEL versions earlier than 7, which do not have systemd at all.

- Resolves: rhbz#2157788

beep.spec

@@ -65,7 +65,7 @@ Requires(pre):  shadow-utils
 # /etc/modprobe.d/
 Requires:       kmod
 # /etc/udev/rules.d/  and  /usr/lib/udev/rules.d/
-%if 0%{?rhel} <= 7
+%if %{defined el7}
 Requires:       systemd
 %else
 Requires:       systemd-udev