Fix build on mingw32

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

net/tap-win32.c

@@ -736,3 +736,8 @@ void tap_set_offload(VLANClientState *vc, int csum, int tso4,
                      int tso6, int ecn, int ufo)
 {
 }
+
+struct vhost_net *tap_get_vhost_net(VLANClientState *nc)
+{
+    return NULL;
+}