tap-win32: stubs to fix win32 build

Add missing stubs to win32 to fix link failure.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

net/tap-win32.c

@@ -752,3 +752,13 @@ struct vhost_net *tap_get_vhost_net(NetClientState *nc)
 {
     return NULL;
 }
+
+int tap_has_vnet_hdr_len(NetClientState *nc, int len)
+{
+    return 0;
+}
+
+void tap_set_vnet_hdr_len(NetClientState *nc, int len)
+{
+    assert(0);
+}