vhost: fix build with kernel < 3.5
vhost: fix build with kernel < 3.5

VIRTIO_NET_F_GUEST_ANNOUNCE is a new feature introduced since kernel
v3.5. For older kernels (or more precisely, old distributions), we
could simply define it manually, to fix the "macro not defined" error.

Fixes: d293dac8f30e ("vhost: claim support of guest announce")

Signed-off-by: Yuanhan Liu <5dcef21195e64c9c08aa779bb62b25e9133eaa33@linux.intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk