[libvirt] Fix -Werror build error regarding to '#ifdef IFF_VNET_HDR'

Hi! Our build bot found a build error when building libVirt with -Werror introduced in b4f62abbf1191c8fbab3306b4bf2f2567e18067f kvm/virtio: Set IFF_VNET_HDR when setting up tap fds The attached patch kind of fixes the build error but I'm not sure, if this is the final solution for this problem, though... Ciao Max -- The real problem with C++ for kernel modules is: the language just sucks. -- Linus Torvalds