build: require avahi instead of avahi-libs on rhel5.

On rhel5, libs of avahi are packaged into avahi instead of avahi-libs.
Actually, there is no avahi-libs package shipped with rhel5. This patch
fixes this by requiring avahi on rhel5.

libvirt.spec.in

@@ -615,8 +615,12 @@ Requires: module-init-tools
 # for /sbin/ip & /sbin/tc
 Requires: iproute
 %if %{with_avahi}
+%if 0%{?rhel} == 5
+Requires: avahi
+%else
 Requires: avahi-libs
 %endif
+%endif
 %if %{with_network}
 Requires: dnsmasq >= 2.41
 Requires: radvd