Add missing ATTRIBUTE_UNUSED

to fix the kFreeBSD build.

The network parameter is unused in networkCheckRouteCollision:

    http://honk.sigxcpu.org:8001/job/libvirt-build-debian-jessie-kfreebsd64/

src/network/bridge_driver_nop.c

@@ -21,7 +21,7 @@
 
 #include <config.h>
 
-int networkCheckRouteCollision(virNetworkObjPtr network)
+int networkCheckRouteCollision(virNetworkObjPtr network ATTRIBUTE_UNUSED)
 {
     return 0;
 }