spec: require libvirt-daemon-driver-interface only when built

Signed-off-by: NEric Blake <eblake@redhat.com>

libvirt.spec.in

@@ -420,7 +420,9 @@ Requires: libvirt-daemon-driver-vbox = %{version}-%{release}
 Requires: libvirt-daemon-driver-nwfilter = %{version}-%{release}
         %endif
 
+	%if %{with_interface}
 Requires: libvirt-daemon-driver-interface = %{version}-%{release}
+	%endif
 Requires: libvirt-daemon-driver-secret = %{version}-%{release}
 Requires: libvirt-daemon-driver-storage = %{version}-%{release}
 Requires: libvirt-daemon-driver-network = %{version}-%{release}