Ensure GET_VLAN_VID_CMD is always defined

The RHEL4 vintage header files do not define GET_VLAN_VID_CMD.
Conditionally define it in our source, since the kernel can
raise a runtime error if it isn't supported
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/util/virnetdev.c

@@ -672,7 +672,7 @@ int virNetDevGetIndex(const char *ifname ATTRIBUTE_UNUSED,
 #endif /* ! SIOCGIFINDEX */
 
 
-#if defined(SIOCGIFVLAN) && defined(HAVE_STRUCT_IFREQ)
+#if defined(SIOCGIFVLAN) && defined(HAVE_STRUCT_IFREQ) && defined(GET_VLAN_VID_CMD)
 int virNetDevGetVLanID(const char *ifname, int *vlanid)
 {
     struct vlan_ioctl_args vlanargs = {