fix building error on non fedora system

We forget to define with_storage_rbd if the system is not fedora,
or the version is less than 16.

libvirt.spec.in

@@ -77,6 +77,8 @@
 %define with_storage_mpath 0%{!?_without_storage_mpath:%{server_drivers}}
 %if 0%{?fedora} >= 16
 %define with_storage_rbd   0%{!?_without_storage_rbd:%{server_drivers}}
+%else
+%define with_storage_rbd   0
 %endif
 %define with_numactl       0%{!?_without_numactl:%{server_drivers}}
 %define with_selinux       0%{!?_without_selinux:%{server_drivers}}