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: d293dac8f3 ("vhost: claim support of guest announce")

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>

lib/librte_vhost/rte_virtio_net.h

@@ -97,6 +97,11 @@ struct vhost_virtqueue {
 	struct buf_vector	buf_vec[BUF_VECTOR_MAX];	/**< for scatter RX. */
 } __rte_cache_aligned;
 
+/* Old kernels have no such macro defined */
+#ifndef VIRTIO_NET_F_GUEST_ANNOUNCE
+ #define VIRTIO_NET_F_GUEST_ANNOUNCE 21
+#endif
+
 
 /*
  * Make an extra wrapper for VIRTIO_NET_F_MQ and